What was stated above is kind of correct. You can double or triple your miles at check-in at the kiosk or online. These miles usually cost about $0.02-0.03 per mile. However, in order for them to count toward premier status, you have to add the premier accelerator, which costs more like $0.10-0.30 per mile. It gets much more expensive later in the year when people know how many miles they need to hit the next level.
You don't actually have to fly the trip to get these miles. Just book a ticket, wait 30-60 minutes until it gets ticketed, go back online and purchase the accelerator miles, wait a few hours for them to get credited to your account, and cancel the ticket within the 24 hour cancellation that United allows with no penalty. That being said, your 2000 miles would have probably cost you about $500-600 right now.