A few tips for dark side and mp management:

1. Remember that you can always click off dark side between pulls to restore MP.

2. At 35 you gain an ability that restores MP based on damage you take. One thing I will do in lower level content, is pull a bunch of trash, spam unleash with the mp restore ability up and by the time it wears off I have a huge threat lead. From there you can use do some Syphon Strike combos to restore your MP since you should have enough threat, of course check the enemy threat meters in case you need to do a threat combo instead.

3. Pre-35 you can play drk exactly like tanking on a low level pld. I.E. low level pld tanking is mostly just spamming flash for threat and using riot blade combos to restore mp. For low level drk you can spam unleash and use syphon strike combos to restore mp.