Learn your boat, learn your boat, learn your boat. Learn to do most if not all of the repairs needed for your boat, and have the tools to do them on board with you.
Set up a rain water collection system, that is not contaminated by salt water. Learn what types of easily obtainable sea plants are edible. Have a serious fishing kit on board, with good salt water tackle and poles/reels. Convert all interior lighting to LED's if you can, will save on battery power. Figure out what power you will need for your fridge/freezer or how long ice will last in your cooler.
If using propane to cook, make sure you set the stove/tanks up properly for a boat and be sure to have a explosive fume alarm. Alcohol stoves will burn rubbing alcohol, and even grain alcohol in a pinch.
You might want a Bimini top or sun shade, as the sun WILL be a bear long term.
Make sure your tender/dingy is in very good shape, and all of your rigging is top notch. You are looking at a used boat, so have a GOOD marine mechanic do a full survey.
There are many books out there about long distance crusing/living aboard boats, check out the library/ebay. They may help you with long term living on a boat. Boats are small in the living area over long term, and will seem even smaller when you load your gear in them. Can you comfortably live in 1 room, because that's about what living on a boat is??
Find a good marina, and spend some time there, getting to know what they sell, what they can or can't fix, and ask questions of other boaters AND the marina staff.

I've been looking at long term living on a boat also, and these are a few of the things I've come up with.