You can get 4 of your friends (they have to be college students) to sign-up for a United Mileage Plus card and get 500 miles for each (they limit it to only 4 per year). I did this, and got the miles in about 3 weeks. This promotion is posted at United's Mileage Plus College website.

The college credit card they have is also ok to get miles (3K for signup and 1 mile for every 2 dollars you spend), and no annual fee. When you get in college you'll be bombarded with credit card offers, but the only thing they'll offer is a lousy t-shirt.

Also, it is looking way ahead, but don't forget that you'll get 10K miles just for graduating college.