I've been through it a few times, and here is what I have come up with...
1) Look at a bunch of websites which have reviews and see what professionals and consumers think. I like AudioReview.com, but not all the newest models are there. Cnet/PCMag, Amazon, Epinions, ConsumerReports, and now even user reviews from BestBuy and Circuit City are helpful. In general you will find some companies almost always have great products (like Pioneer Elite and Mitsubishi), whereas others are hit-or-miss (Samsung is a perfect example here). There will always be one or two oddball reviews, but go with the trend (ie, Brand X seems to always be bad or Brand Y usually seems pretty good).
2) Even if you do not plan on buying in-store, go to the store and look at them. If it has an amazing picture, you know what it is capable of (and their sources may be HD-DVD, or other high quality signal, so you will need a good signal to get a good picture). If the picture is not high quality, it may be the TV itself, but it also may be the setup (poor signal, loose connection, not adjusted properly, etc.) If you see a TV that looks bad consistently from place to place, then you can think it may be the TV itself.
3) Protect your investment. I got screwed with my TV a few years back. I purchased a nice 32" TV brand new. Within three years, a lightning storm killed it, and the cost of fixing it was worth almost as much as the TV itself. I know extended warranties can be a rip off in many cases, but if they cover damage like that or accidents (ie, it gets damaged if you move), it is well worth it to have. Also, I got myself one of those MonsterCable power stations with damage coverage on it.
4) Have somebody that knows something about TV's help you set it up... Adjusting brightness/contrast optimally, calibrating it, etc. all for your viewing conditions will help you get the most out of it.
5) If you shop at BestBuy or Circuit City, or any place like that, open box deals can save you money sometimes. If they are missing the manual and remote, those are easy and cheap to replace. Just make sure the warranty coverage is appropriate and the TV itself seems to be fine, and that the return policy is acceptable to you.
6) Budget for other things you need... After all, that new 1080p TV will need friends, like an upconvert DVD player, HDMI cable, receiver, etc.
Hope that helps.