Don't crew for these guys, they don't maintain their boats for crap. One of them went down a few years back in a hurricane, lost the entire crew. As far as crewing opportunities go look up crew seekers, crew finders, post on all the different sailing forums, and the way I got my first job on a boat, look in the back of all the sailing and boat magazines you find. As far as avoiding the captain bly's out there, the yachty the boat the pickier the captains. Look for either a working vessel, like Passenger carrying tall ships, charter boats, and such. These are going to be more likely to pay you something any way, you might be able to hang out in the carib, just working as a mate for room, board and tips. When you want to go some place new, look for a decent boat that isn't to shiny, a couple of scratches here and there tell you the boat is a moving boat and not a shrine to the sea gods.
I'm going to list the next in increasing order of opportunity to avoid sailing with an ass.
1. Good. If you find a trip that starts out with a couple of short trips before a long one, ie.. a couple hops up the eastern seaboard before crossing the Atlantic.
2. Better. If you find a trip that starts with about a week of yard/refit. If some is prone to blow his top, or is overly picky with the help so to speak, it will show up in yard alot faster then on the water. Plus you can leave at the end of the day, you don't have to wait till the trip is over.
3. Best. A combination of 1 and 2. This way you get to see their temperament and get to see their seamanship before your to far out to hop off the boat.
A couple of things to remember. Working vessels pay but rarely go far from their home port. They are nice to the help though. Private vessels that pay are usually owned by some picky buggers and are usually more or less manned by full time and trained crew, so unless you want to be the cabin boy, etc... Private vessels that don't pay are normally own by cruisers. They are going cruising or they couldn't get free help. They're more likely to be going some place fun, and are nicer to the help.
A good example of how to get started is of a fellow crew member I worked with. He started as a fish mate on a charter fishing boat in Key West, then came to work with us on the Schooner Liberty Clipper, also in Key West. After a couple months he met a guy with a 38' sloop going down to Cuba. the guy had single handed down to the keys but want company to cross open water. I heard from him after he had left Cuba and wound up in Mexico. I lost touch after that.