Always keep your weapon (and shield when able) within 5 levels of your actual level. Gear within that is important, but the weapon is the main thing.

After marking a target, if the DPS isn't focusing that target, tell them to follow marks. ie 2 dies before 1. If they complain about it, tell them if they pull it off you they can tank it til 1 is down. My fiancee used that on one horrid group, after the first time they tanked it; they started being better at following marks.
Also, synced players even though synced, have higher stats then those naturally that level; this causes them to hit harder (depending on what their syncing from). While they won't like being asked to back off the dps a bit, MOST will.