Getting good grades should be obvious. The better your grades, the more doors you'll have open to you. You may never choose to go through those doors, but more options are always better than fewer options.
The advice I wish someone had given me is to take advantage of literally everything your university has to offer. Take the class in that strange subject you've always wondered about, develop relationships with professors and seek their mentorship, go study abroad, join the ski/surf/sky diving/whatever club, do stuff that takes you out of your comfort zone, learn a foreign language, start playing an instrument, etc. You'll never have the abundance of time and opportunities you have now. So don't waste it watching TV, playing video games, fiddling on your iphone, and sleeping off hangovers (at least not too often).
