I moved aboard my boat a couple of months ago. There's already a wealth of good advice above, so I'll try and be succinct.
1) Look at boats in the 29-36' range, maintenance doesn't scale up with length, it scales up with area. A 36ft boat isn't half again as much work as a 24ft, it's closer to double.
2) Smaller is also less boat to clean and keep tidy.
3) Smaller is also less berthing fees. My marina has a completely different (and much higher) tariff after 10m.
4) Don't start by taking all the luxuries, hobbies and possessions in your life and trying to see what you can lose to fit them into a boat lifestyle. Instead start at a blank, figure out the bare minimum of what you need to life comfortably and then see what you can budget in afterwards. Build it up slowly, carefully assessing each item, be brutal.
5) Read these two threads Living Aboard Cheap
and Boat Inspection Trip Tips
6) Get out and look at boats, look at as many boats as you can find in your price/size range. At first you will see a lot of awesome boats, try not to get over excited. Agree to nothing without sleeping on it.
7) Find likely marinas, get details of their tariffs. If you're a little geeky, like me, throw together a quick spreadsheet, adding each possible boat and adding on the insurance, berthing costs and the cost of any work that may need doing, be generous. Compare it to your expected income, the less money you spend on the boat the more money you have for luxuries, trips and savings etc.
8) Look at some more boats. You'll know the one when you find it, I did
9) Get it done, I'm wondering now why it took me so long.