You talking about just coming in or being in? If you are just coming in, a degree with get you E4, knowing basic soldiers skills and referring friends can provide the recruiter with the ability to advance you to E2 or E3. Now if you are talking E5 or O series. It's more like being in the right MOS at the right time and knowing the right people. Because as it stands the smart people seem like the ones who are getting out and the ones that can do push ups and sit ups are the ones advancing. Nothing like a Armed Force full of physically fit meat heads.
in the USMC its all based on a score of MCI's, PFT, rifle score, and pro's and con's after E3. E1-E3 is just based on time though. you cant get E4 in the USMC for college credits though b/c thats already NCO. after that im not to sure.