Know the mechanic giving the pre-buy, and trust him, or use one highly recommended to you by someone you trust who is NOT the seller.
A prebuy thorough as an annual is not a bad thing. In fact, if you want to really get into the guts of the plane and examine it, remember that an annual is really only the inspection that determines what needs to be fixed to be airworthy, not the expensive part where it's all fixed. If the plane is out of annual, you may even get the buyer to split the cost of said inspection with you.
Know the common Airworthiness Directives for the plane, and the number of ADs overall. Make sure the mechanic knows the AD's for the plane, too - and that they've been complied with. If they haven't been complied with, well, if it's an expensive new AD, you may be able to negotiate the price based on known cost to comply. If it's a major AD that's older and the plane's obviously been flying without compliance, be prepared to need to go over everything with a fine-toothed comb.
Find the owner group, and ask them for advice. They can provide lots of very useful advice specific to that particular airplane, from availability of parts to things to specifically check for.
Stand back about 30 feet from the airplane, and look at it. Are the wings both level and at the same angle? Is the frame straight? Are the landing gear legs both set with the same toe-in? Does anything look a little off? Walk around the plane, again about 30 feet away, and just look at it overall.
If you find something wrong, keep looking.
Know that if you buy a fifty-year-old plane, chances of finding one with no damage history are very small, and even less so for an eighty-year-old plane.
A perfectly preserved 1970's paint and interior tells you either the owner babied the airplane, or always hangared, never flew. A worn-out but well-repaired interior and faded paint job tell you a lot about the care and keeping of the plane. A brand new paint job tells you nothing, good or bad.
Do not get a project plane if you really just want to fly; that's as disastrous as moving into a fixxer-upper house to live. If you want to learn to work on your plane, to modify and tinker and build skills and have a flying airplane further down the road, a project plane is great.
Make sure any unfinished experimental project comes with all original documentation, and documentation of the work done so far. Expect that you'll want to tear out and redo things someone else did that made sense to them, but not to you.
If you want to buy an airplane, start saving money now. The more money you have in the bank - and the better your habit of saving money each month - the more you will have toward the plane you want, the repair you need, and the cost of going flying once you've drained your wallet by buying the plane.
As for myself, my flying buddy came back from the Valdez May Day Fly-in and told me that he'd seen a project Taylorcraft for sale, just looking for some love and attention. He's very tall - I almost come up to his armpit - and he grinned down at me as he said, "I saw it, and I thought, 'That's a you-sized airplane!'"
I asked my IA if he thought I had the capability to tackle a project airplane, and he recommended I look at one right here on Merrill Field. With his advice and guidance, but using a different A&P for the pre-buy, as my IA had worked on this plane and wanted to avoid any appearance of conflict of interest, I looked it over, took a deep breath, and tackled the project. I was able to negotiate a downpayment and paying a thousand a month to the seller, so she's mine free and clear - it was a little painful on a retail salary, but who needs to eat out or new clothes anyway? Two years and several distractions later (relearning how to walk, getting married), I'm nailing the ribs and looking to fly within a month.
It's been a long and interesting road, and I know a lot more than when I started. I'd do it all again... in fact, it was awfully hard to remind myself that I should get this one flying first when an opportunity for another project T-crate came up last week. I still delude myself by thinking I'm not going to get my A&P - at least, not for another few years - well, maybe just not right away... Yeah, right!