- Grades. If you're planning on continuing on after undergrad, you'll probably want to spend some time working on these (more so than the average student I see, at least!). There's no set GPA or anything for all programs, but a steady stream of C's and below probably is going to hurt your chances.
- Internships. Find out what you want to do and find the time to actually spend time DOING IT. Or as close as you can. Work in a lab, a law office, go on that archaeological dig, spend time in a hospital, and hope you love it. Love every freaking second you are out there. If you have any doubts about loving it, well, you might need to take stock about what you're doing.
- Learn how to write. This is something that I wish most college students would ENTER college knowing how to do. It would have saved me a whole lot of headaches over the years of TA'ing while in grad school, and is probably the singularly most important skill you can take with you into grad school. Because you'll be writing a lot. Papers, reports, and academic articles (this will vary depending on your program, obviously), it never ends. I had to learn this the hard way, thanks to not attending school in English until I was older, and it's been really frustrating. No one likes getting papers back dripping red ink :)
Okay, anyone got any others? I know I'm probably missing a bunch of stuff!