Through a recruiter, they take almost anyone. Don't do drugs, be able to run 2 miles at a decent pace (under 18 minutes). When being offered a job at MEPS, don't settle for whatever they tell you. Go in knowing what ASVAB and GT score you need for certain jobs and be firm about what job you really want.

MEPS is a nightmare, especially going from active duty to the guard. Swallow your pride and be prepared to be stuck there all day, no matter what time you get there.