Brake abuse is a good example of a bad habit. I will tell you that the 75% rule is a bit outdated though - modern disc-brake bikes can do 100%... and beyond.
These examples may be a bit more advanced in some cases than simply riding aroudn the yard and driveway, but some other things that folks can get into trouble with:
- Keep loose. Your entire torso, shoulders, arms, and grip should be relaxed at all times. Tensing up makes things worse in about 2 dozen different ways.
- Looking at the road just ahead of you, not looking far enough ahead of you. You are moving further down the line than the patch of tarmac in front of your tire. Looks where you want to go. If you want to go to the end of the block, look to the end of the block. Get the big picture. Look down, go down.
- Thinking your mirror gives you an idea of what's going on behind you. A mirror can only be trusted to tell you it's not safe, it can never be trusted to tell you that it IS safe. Check blind spots, be aware of what's happening behind you as well as in front.
Just a few examples. You may be doing them already and not realize it. Even if you're in a class you may be doing it but not realize it. Luckily your trainer knows how to spot it and coach you on it.
Don't over-do it on your own. It can also make you fearful of the bike because you encounter an issue that you cannot overcome entirely on your own, which will make you more anxious when you get to class and will give you mental blocks on some things.