#1 Talk to Your Significant Other
One of the biggest issues I see constantly are folks who go buy a kayak without discussing it with their spouse, boyfriend, girlfriend or fiancee first. Obviously this really only applies to those that have shared bank accounts or shared financial responsibility but a good discussion of why you want it, how much it will cost and what it will be used for will lay the groundwork. I see a LOT of big box store kayaks back on the market a couple of days after purchase. This is because big box stores usually don't take returns on kayaks. Communication on a large purchase is always advisable.
#2 Demo, Demo, Demo
You are tired of hearing it by now I am sure. If at all possible demo a kayak before you buy it. I'm speaking from experience. I'm a knucklehead and have broken this rule MORE THAN ONCE. Find a buddy with the kayak, take a drive to a shop, do something to make sure that the kayak you are about to drop that hard earned money on is actually the right fit. Save yourself some heartache.
#3 Try to See Through the Hype
Major brands are good at marketing. That's how they become major brands. Don't buy a kayak because you want to have the same adventures as the people in the commercials and videos. Don't buy a kayak because you think it will get you into a new peer group. There is no perfect kayak. If you ever hear someone say Brand X is the perfect kayak, they either a) are speaking only for themselves or b) are a liar. Only you can decide what the best kayak for you is. Frankly, it rarely is the first one you buy.
#4 Make a List
Sit down and make a list of all the things you want in a kayak. Make a list of all the places you'll be fishing. Keep the list realistic. I bought a kayak once because I wanted to fish BTB (Beyond the Breakers) more. It didn't result in anymore trips than normal and didn't get used for what I bought it for. It wasn't realistic. I fish lakes and rivers 10 times more than saltwater. It's a proximity thing. If you are nine hours from the nearest beach, buying a kayak specifically for BTB might be a bit pie in the sky, especially if it's the ONLY kayak you will have. Ask lots of people what they like about their kayaks. Then you need to ask them what they would change. Take all of this with you when you go shopping.
#5 Don't Rush
The little kid in me has come out with a few kayak purchases in the past. I'd sell of some gear, get $250 in my pocket and go buy just what I could get. Literally less than a week from when I had the money in hand, it was gone. What I should have done was take my time, follow all these other tips, save some more money and gotten something I would have liked for the long term. Don't buy a one night stand kayak. Look for the long term relationship.