Take classes that you enjoy!
Undergrad is a time to work hard, but you don't want to burn yourself out by doing nothing but studying.
It looks better in an application process to have diverse interests and it'll make your undergrad career more fun too! Sure, it's great to join the pre-med/dental/pharm society, but that won't make you stand out and you can't be all pre-med/dental/pharm all the time. Try the breakdancing club or the underwater basket weaving enthusiasts group if they sound interesting to you.
Be nice to your TAs. They work hard (and a lot of them really dread the pre- students' emails after exam time).
If you don't enjoy your major, change it, even if you don't think it'll look as good for your application.
Your health is really, really important. Make sure you sleep enough, make sure you eat enough, and make sure that you have your stress under control.
Don't sweat the small stuff. A B won't ruin your chances of getting in. Neither will two or three. Just do your best and that's all you can do.