As explained before, the game doesn't reward you enough Triad XP to reach the highest level by the end of the story line.

This guide gives: 1) General tips and tricks & 2) Mission specific advice and exploits

The second obviously may contain lots of spoilers!!! - you have been warned!

so you can avoid the repeated re-playing of missions after the end of the storyline, only to get those extra little amounts of xp until you are level 10.

Wenn mich genug Leute darum bitten, würde ich den Guide auch nochmal auf Deutsch verfassen.

Okay, now it gets interesting:

First some caveats and advice.

• There are more than enough face xp and cop xp in the game to become level 10 in both of them long before the story is finished. So never worry about those. This is important in regards to tip No. 1.
  
  
• If you have the "Low violence"-version of the game, this guide will be almost definitly be a necessary evil to come even close to getting the full Triad Xp. This is a design oversight, which is very annoying. Coupled with the fact that some DLC give you hefty amounts of XP, it accounts for some lazy design which is annoying and detrimental to a good game.
  
  
• Some of the strategies for earning xp are a bit counter-intuitive. That has nothing to do with my guide, it is just how the mechanics of the game work.
  
  
• If you follow my general tips, the mission specific advice shouldn't be necessary.

Okay, now come the real Tips and Strategies

It is very tempting not to wear the "Triad Bonus XP" Clothing, because the cop clothes almost always look better, and the "Thug" bonus damage is so very nice. But there are several reasons why you definitely should wear it.
Bonus XP is only awarded to NEW xp earned. So you can't simply replay the mission with a different set of clothes and get the full 15% bonus on the first few missions. As soon as you can get bonus xp through clothing, go for it. It will save you lots of trouble later on. Don't be afraid to spend a whole lot of money on the expensive "Mountain Master Set". It gives 15% bonus, which is immensely useful .

Fighting or Brawling is one of the main sources of Triad XP in the game, so there are different bits of advice:

First for Hand-to-Hand

1) Mix and match
If you don't mix up your fighting moves, you will get reduced xp. So for example, the first counter-attack gives 15 xp, but if you follow up wiht another counter, the second will only award 10 xp. And so on and so forth.

2) This leads into this advice: The more brutal and heavy your attack, the more xp you get. This is the main reason the "Low violence"-version offers less xp. They simply cut out some of the more violent&nasty kills. Like impaling people on narwhal horns, or mashing their heads into table-saws. If you see something flashing in the background, like speakers, aquariums, dumpsters, etc. GO FOR IT!

3) Try to get the most xp out of every opponent. If you kill an opponent straight away with an enviroment kill, you get about 20 xp out of him. But if you counter his attack first (+15) follow up with a fly kick (+15) and then you put him in the dumpster (+20) you get almost 3 times the xp out of a single opponent.

4) Practice, practice, practice
If you want the most xp out of storyline missions, which are outside of DLC the only way to earn Triad XP, you need to practice hand-to-hand fighting. A great way to do this are the "Martial Arts-Schools" or Fight Clubs. Especially the first one in North Point offers a very manageable challenge, 10k HK$ for every time you win the full five rounds, some face xp and NO penalty for loosing. Use the fight clubs to practice the heavy moves you learn at from collecting statues and the moves you get from gaining Triad Levels. Don't use drug busts for practice. If you loose those fights, it costs a lot of money because you "die".


Second comes Gun Fights

Gun fights are a lot easier in terms of xp-gain, but often harder in terms of gaining "perfect" scores.

1) Headshots.
Simple, really. Regular kill awards 10 xp and a headshot awards 30 xp. Do the math.

2) Disarm!.
A lot of thanks to TrenchaunT for giving me this idea.
Basically: headshot: 30xp - disarm: 30xp - disarm & then headshot: 60 xp.
If an enemy is close enough to grapple or to sprint grapple him and he has a gun - do it. Twice the xp.

3) Enviroment kills, again.
See someone standing in front or behind a gas canister? Shoot it. Explosive kill: 95 xp!!!
Sadly, they tend to be bugged, so this doesn't always work. In doubt: headshot.

4) Shoot up everything belonging to the enemy.
Shoot their car: 10xp for reasonable damage.
Blow up their car: 25xp + the 10 xp for the damage.
Tire shot: 30 xp.
If you shoot at the hood of cars, they catch on fire and explode after a while. It is quite hard to get explosive kills that way, since enemy AI usually runs away from burning cars.


Car chases

Now for the hardest and simoultaneously most rewarding part of the game.

1) Same as rule 3 from hand to hand fighting and gun fights: Shoot them to bits, preferably before killing them off. The more xp you get out of a single enemy, the better.

2) Ramming damage also gives you reward xp for your triad score. Never mind the cop xp. You can gain that on a later re-play of the game. But you shouldn't really worry either way, since getting cop xp is very easy through drug busts.

PRO TIP: The most rewarding thing in terms of xp in a single magazine of rounds is the following maneuver. I call it the "Flying dragon" ( not really ;-P )

Shoot up the car of the others, while aiming only for the windows and the heads of the passengers. Gives xp for headshots, enemy down and damage to vehicle.
Then shoot their tires, gives xp again. BUT: try to do this when the enemy car is alongside yours.
Now comes the hard part. When the car flips, aim for the bottom of the car in the area of the trunk. If you get enough hits in, the gas tank and car explode in a ball of fire. Extra xp for vehicle destruction and for any remaining enemies that were still alive inside the car.
If you follow through with the whole maneuver, its like 100~250 xp in one flying burning car.

SPOILERS !!!

Okay, this section is waiting to be expanded, since I surely haven't come across EVERY exploit there is. So if you've got any nice glitches or exploits to share, feel free.

First the full list of missions available for replay in the social hub.

Vendor Extortion
Night Market Chase
Stick Up and Delivery
Mini Bus Racket
Club Bam Bam
Payback
Uncle Po
Bride to Be
The Wedding
Mrs. Chu's Revenge
Meet the New Boss
Loose Ends
Final Kill
Initiation
Dockyard Heist
Intensive Care
Important Visitors
Fast Girls
Bad Luck
Conflicted Loyalties
The Funeral
Civil Discord
Buried Alive
The Election
Big Smile Lee

Now the list of exploits (no spoilers, only those missions that have known exploits):

Vendor Extortion

The first guy you are supposed to beat up is coded to be killed with and enviroment kill. If you simply wait and get in a few counters and heavy hits before you do that: extra xp.

Mini Bus Racket

Try to knock out all of the enemies before your crew does it. Your crew isn't afraid of kill-stealing.

Club Bam Bam

Enviroment kills, lots of enviroment kills!

Uncle Po

Besides doing the "flying dragon" on the escorts of a certain lady, don't be afraid to shoot up her car a little bit (not too much), for those few extra xp.

Meet the New Boss

Instead of running from the cops, take them down (Sadly its not possible to grab their guns). Extra triad xp and no real punishment in terms of cop xp.

Special strategy (thx and kudos to TrenchaunT)

1. When you start, shoot the first enemy in the head. Run toward the new two, but holster your weapon and press grapple to disarm one for 30 xp. Quickly hold aim to start slow motion and shoot him in the head. Then holster again and shoot the other in the head. These first three enemies have given you 150 xp.
2. While still in slow motion, run to the railing and shoot the two nearest enemies in the head, before your allies kill them. Jump down and shoot any enemies with heavy weapons in the head. All of the others, holster, disarm, and then shoot in the head. This way most of the enemies will give you 60 xp instead of merely 30. You can also do a heavy strike on an opponent before you disarm him for 15 more xp, but this can be tough without dying.
3. If you fail to shoot any enemies in the head in this area, you can restart until you get it right, but doing so will remove any bonuses from tea, food, and massages. Note also: Avoid using the SMG for headshotting enemies, because it is terribly inaccurate. Use a pistol or an assault rifle.
4. When you leave the bar, start by shooting the hood of the car where Jackie was until it blows up, for 25 xp. Then shoot any enemies in the head except for one, and destroy the cars near them, making sure not to let the explosion kill any, because most likely it will only give you 10 xp if that happens.
5. When reinforcements from the right come, shoot them in the head and then blow up the car. Blow up the car you started behind. Then look to the left for a car to drive in. Try to shoot the passengers in the head as it comes on, then blow up the wheels, and maybe even blow up the car. As long as you left one enemy alive, you will have time for all of this. Finish off the last enemy.
6. Car chase – you know what to do!
7. When fighting Ponytail, don't counter or attack him normally. Just grapple him and break his leg (very hard). Run away, wait 10 seconds, and then break it again. Every single time, you will gain 43 xp. You can do this a good five or so times before he dies.


Bad Luck

Fast talk ALL of the guards for full xp for this mission.

The Funeral

Remember my advice about shooting up the enemies cars before taking them down? This is the mission to do so!

Big Smile Lee

This mission has the single biggest exploit I found on my own: Unload a whole f***ing magazine of assault rifle bullets into Ponytail's head.... XP for every bullet.... The satisfaction.

So, thanks for reading my guide!

Thanks to United Front Games for their fun game!

If you find any mistakes in my guide or feel the overwhelming urge to add something, just comment! I'm happy to add anything people might find helpful.