When I started being premed partway through senior year, I had to give up a few things. I still finished both my degrees, but I didn't go much beyond the minimum requirements, and had to skip a few interesting classes I'd wanted to take. Of course, I could never have taken all the classes I wanted, so I was always giving up one or two. That last semester (4 labs, 21 credit hours) I had to turn down invitations to go out to bars with my friends, which kind of sucked, but on the other hand, how much do you really need to drink. I had to struggle a little in o-chem to not become a neurotic premed, but I think I've done pretty well.
The only thing I really missed out on was a great travel opportunity for work - it was over the weekend with the MCAT, and I had already paid and studied for it, so I couldn't go on the trip.
I try to remember that, no matter what my future plans are, my life is happening RIGHT NOW. Sure, I shouldn't live completely in the moment, max out my credit cards, and spend all my time at the bar, but if I forget to live NOW because I'm thinking about what's going to happen in a year or ten years or whatever, I'm seriously missing out. So I sacrifice a little extra sleep and TV time in favor of spending my time having fun and getting my work done. And I'm probably happier than I've ever been.