Is it possible to continue earning Police or Triad XP after completing the story?

You can continue to gain your Triad XP after completing the story by replaying the Triad missions. The amount of XP given for replaying a mission is equivalent to the difference between your final score and the highest score from your previous attempts at the same mission as long as it's greater than 0. There are clothing sets that you can wear that increase your Triad XP gain (5%, 10% and 15%). Here is a set that gives you 15%:

• J. Jiggler Straight Leg Jeans ($21,620 from Achete)
• Couronne Puffy Vest Red ($28,800 from Bruno James)
• Achete Stand Up Boots Black with Brown Sole ($17,400 from Bruno James)

Both shops are found in Central.

You should, theoretically, be able to get max Police XP before completing the story. In case you don't, you can gain them by replaying Police missions. You might have also noticed that there are optional drug bust missions (light blue police shield icon on the map) dotted all around the map, 17 in total IIRC. These missions will also give you a good chunk of Police XP. You should get the achievement associated with unlocking all 10 police upgrades way before the Triad one.

All missions can be replayed from START > Social Hub > Missions.

I found out today, when I was replaying "burried alive" but if you kill a few guys in the boats,and leave a few after you scape from the police chase. You can kill the rest of the triads, and just simply ram over their boats to obtain 10 or 9 xp per hit I know it's a slow process but you can do this until you gain the amount of xp you need to reach triad level 10. This is not so bad if you have the time to burn :) don't worry about jacky you can ram the triads boat as many times as you want, but remember don't go too far from them or they will disappear. I needed about a quarter of the bar to reach level ten and by doing this it took me about 30 minutes. Not so bad. Now I have my plat because of this!!!! Good luck and happy trophy hunting.