Alright, I'm pretty sure I'm the only one here who actually TEACHES AF UPT, so I'll give you the straight poop. Unlike what a bunch of people above me have posted, having extra flight time WILL NOT HURT YOUR performance at UPT. In fact, it will help, not hurt your UPT experience. So would a school like ATP or one of the others HURT you....NO. It would probably make pilot training a heck of a lot easier, and it certainly wouldn't get you washed out.
However, (AND THIS IS A BIG HOWEVER),
that extra time is NOT a requirement, and you don't need it!!
You definitely don't need multi-engine, turbine, or any other kind of high performance "time" to get through. It isn't going to help that much. You certainly don't need to "liquidate all your assets", and like every one else has already said, and I wouldn't go to a professional pilot school like ATP or anything unless you just have extra money to throw away. These schools wouldn't hurt you, but they are probably a waste of money, since you will be receiving that training again for free (in fact, you'll be receiveing BETTER training, IMHO). Very few pilot training candidates wash out. Those who do, usually wash out because they don't want to be at UPT very much, and therefore don't study enough.
Yes, UPT is stressful, and yes it is a difficult program. When you are here, you will need to devote a lot of time to studying.... But that's true even for guys who come in with Instrument ratings or previous airline time.
SO INSTEAD.... Save your money, and continue to rent cessna's or kitana's or whatever you've already been flying and go out on those $100 hamburger trips. Practice stalls and falls, fly to places you've never been before, or if you have a lot of time before you start UPT, maybe get working on your instrument rating.
Or don't... Most of the student's I teach have 50 hours or less of total flying experience in their entire lives, and they do just fine. I got through UPT with less than 20 hours before I showed up. What you really need to do to pass UPT is to devote your entire life to the program from day 1 until the day you graduate. Study hard that whole year, and you will do just fine.
Lastly, if you REALLY are looking for a head start, you need to look at things that are T-37 or T-6 specific (depending on which training program you are taking). If you are going to Laughlin or Vance you are going into the T-6. Going to Columbus it's a 50/50 chance between tweets and T-6's for the next 1-3 years, and then it's all T-6's. If you are looking for stuff to study check out
www.baseops.net and look at the column on the far right of the website. They've got some good stuff there about both the tweet and the T-6, but some of it is old and some of it is incorrect.
AND SAVE THE LIFE SAVINGS... The USAF doesn't pay well enough to just be throwing that money away.