General:

-Know what food to use, try looking at sompage for advice.
-As Garland said, know your camps and get at least 6+Silent Oil/Prism Powders(even i do it as whm).
-Know your macros, make them only showing whats needed or if theres roleplay NOT SO MANY <calls>
-Know your abilities, which one draws hate the most and etc...gauge hate, its not only the tank's responsibility.

DRG:

-Some basic things i learnt to know is while playing on my brother's DRG, is to NOT Jump when you have over 100TP. Jump is a good way to gain TP, getting more TP over 100 is usually not that useful...its always a race with your skillchain partner to 100.

-At lvl 30 you get Acc Bonus which is +10(one lifebelt). So keep that in mind and dont to TOO crazy on Accuracy, you have something backing you so focus on ATK too. Pure Acc build DRGs dont do that well from my experience.

DRK:

-Contary to the above, a pure Acc build DRK is not as bad as a build Acc build for DRG. Because DRK gets 3 bonuses to ATK, hence +30 ATK. That doesnt mean neglect ATK, but you get bonuses to it so its less of an issue.

-Dont neglect your magic, Dark skill with Stun will go up on its own, however i highly sugguest you lvl up your Enfeebling too, for Sleep and other stuff, it might not be that useful in Exp, but in many situations where it might count, you will be glad. Dont slack in spells if you can.

-Dont use Absorb TP recklessly and learn how monsters get TP and use them.

-Level both Great Sword and Scythe, dont skimp. However dont use these weapons as a WAR(use GAxe/Axe...preferably GAxe).