Disguise is a big one because of how useful it can be for killing as well as the bonus XP - remember to select the Disguise objective on the wheel before actually doing it - those small bonuses add up.
Well-placed Caltrops rake the XP in pretty well. Also, running defense objectives when you don't have something better to do helps A LOT. Just stay in the area and get XP for it.
What body type do you use? I'm Light, so I rely on speed and fluidity to get from objective to objective most of the time. When you're Medium, you can more easily guard an objective, and even better still with Heavies - but you can't move between the better-scoring objectives as efficiently as a Light body does. The best tactics are based not only on your class, but also body type. Keep that in mind as well.
I have to duck away from defense objectives a lot because if I take a few hits, I run out of health, and not dying is often important when you're defending something.
Of course, that leads me to another point - don't be afraid of death - if there's no Medics close, or they're bogged down by enemy fire, just go into the next spawn wave. If there's a Medic, but you're surrounded by enemies, take the wave - even if they've thrown a syringe. If you're only a couple of seconds from succeeding in a defense, but there's an enemy about to disarm your explosive, or whatever it is you're defending, a suicidal rush will often be enough to either gun them down or draw their attention - either way, you finish the objective and the enemy fails. A glorious death in completion of an objective may not actually get you any glory, but it tends to get you that defense XP. Also, it moves you one step closer to the next opportunity to have your own objective to complete.