This comes off frequently as such a cliche, but is so true. To expand on it, my tip would be to make sure your members are comfortable with their roles. There are few things less enjoyable in this game, IMO, thank playing with someone who is uncomfortable with their role or, worse, doesn't like it - like the hunter who can't find a group and pugs his healer, but gripes about other dps'ers or QQs about healing the whole time. Your tank should be okay with handling the pressure associated with tanking, your healer should be cool with the priority of healing (pre-heals on tank, heals as needed on dps'ers without healing capability, triage only on slef-heal dps'ers), and your dps should understand what they can and can't do (like rules about fear, hunters who leave growl on their cats, rogues who can't or don't sap on cue). A well-run instance should result in a learning experience - something to build on for future runs - and that's often not the case when people don't feel good about their role.

I, for one, don't think you benefit from running lower-level instances than you are - the challenge, to me, is not as palpable - I learn more when I know a bad pull is going to create a wipe. And if you haven't done Ramps yet, watch the cluster at the top of the stairs - it always ends up worse than you plan :-)