The best way to get your foot in the door with the NPS is an internship through the Student Conservation Association (SCA). They have all kinds of internships with the federal land managing agencies, meaning not just with the park service, but with USFS, BLM, and USFWS too. You should decide what you want to do with the park service, and then get an internship similar to that. There are alot of options, from interpretation (giving programs to visitors) to wildlife management, backcountry, all kinds of stuff. Everything you need to know is on their website (www.thesca.org), and apply now to get internships this summer.
You definitely don't really make money doing it (although, you do get your travel expenses paid plus a living stipend), but it is a great experience no matter where you go. Plus, it is the best way to get your foot in the door with the NPS, because usually it is alot harder to get a job with them if you don't already have experience doing that job. It worked out pretty well for me, and I've known alot of other NPS employees that started via that route as well.
You can also try volunteering with the NPS, although each park runs their volunteering programs a little differently. Search each park's website (start with www.nps.gov and find the parks you are interested in), and see if you can find a volunteering contact at that park to talk to. Some have more demand for it than others, so it is definitely an option, but as a student I'd say that the SCA is the best way to go.
It worked for me!