Awesome, now I can rant without sounding like a whiner.
I agree 100% with what the others said about being prepared for lessons. I've had this problem bad a couple of times lately, and solved it by saying, "we're not getting back into that airplane until you read that material, and prove to me that you know it." It gets moans and groans, but it works.
Why does it seem like some students can hold 2900 or 3100 perfectly if I say to hold 3000? If you are off in altitude, don't just stay there- GET IT BACK! I once let a student fly for almost 20 minutes at 2900 when he was supposed to be at 3000, just to see if he'd get it back. He didn't even bother until I said something- that drives me nuts. Strive for perfection, don't be happy with a halfass job.
Which brings me to the next thing: laziness. Be proactive, not reactive. Like I said above, if you see something wrong, don't accept it, FIX IT! Don't wait for us to say something. We want to see good decision making from you, and in order to do that, you need to make the decisions without waiting for us to input. Trust me, if its not the right decision, you will hear it from us.
And USE YOUR CHECKLIST. Some of my instrument students seem to think that just because they are private pilots now, or they've got a lot going on at the time, that they don't have to use a checklist. Well, all that does is hurt them. I will sabotage things on the checklist (like put the fuel selector on L or R instead of both) and when they forget, they will hear a lengthy post-brief on how that affects safety, and how they would have busted their checkride for not using checklists. If they don't pick up a checklist once after they've finished their runup, then we repeat the flight- regardless of how well they did. That usually solves the problem, at least temporarily.
Well, thats it for now...I've got a few flights today, so if anything else comes to mind, I'll post it later.