Follow your heart! I know it sounds like a cliché, but the things you are really interested in now, as an undergraduate, are the areas that will most likely still interest you later, too. I ended up side-dabbling in political science as an undergrad out of sheer interest, and while I saw myself in the future as a 18th/19th century history of ideas kinda guy, for which I applied as a grad student, life pulled me simply back into 20th century foreign policy. I switched back into that field only after entering the Ph.D. program, and eventually became (nominally) a Cold War guy rather than the 19th century political philosophy guy I once thought I would be.
I think that graduate programs make a bit of a mistake by wanting to nail you down too early. Do not be deterred by that -- you can still switch to other areas.

As for other general advice (which someone should have given me properly): make sure you network. Network, network, network. Go to conferences, meet people, stay in their address books, keep them in yours. It all pays off later. I was almost autistically only interested in knowledge, and not so much in building a professional network, but it is what will carry you through. (My wife is tenured -- I left academia eventually).