There is much advice out there on the web, but here are my views on cruising with strangers. Please use your own judgement.
1. Decide on where, when, with who, and for how long you'd like to go sailing.
Do you have two weeks off from work? Or, are you quitting your job, selling your stuff, and taking off indefinitely (like I did in 2011)? Obviously if you have only two weeks you probably don't want to take a 22 hour flight to a remote location, and back, and use up your valuable vacation time. Also, depending on the boat and the weather, they might not be able to say where they'll be at precisely what day. You might look for opportunities closer to home, like I did, in the Caribbean and Mexico (or in the Med if you live in Europe). You will learn just by reading who is sailing where and when, based on wind patterns.
Also consider what sort of people you want to sail with. There are plenty of single captains out there, and also families that might need a hand with the kids, younger groups that might only want to sail with younger sailors, couples that need an extra hand or night watch woman. Who you sail with can be the difference between the trip of a lifetime and a nightmare.
Next, consider what kind of boat you would enjoy sailing on. Size and amenities matter. If you would feel uncomfortable without your daily 20 minute shower you might be unhappy you chose a boat at all. On smaller boats you can generally feel the motion of the waves more than on longer ones, and cats are the most like a floating house. Does the boat have a dingy with an outboard? Or are you going to be rowing to shore? Will you have your own cabin? Is there an autopilot? How many on board? Is there a refrigerator? Important answers to know. Assume nothing.
2. Put up your profile on these sites and look at others'. There are other crewing sites out there, but these are the ones I have used.
When you write your profile it's important to be honest about your abilities. If you've never been sailing, say so. Not everyone needs experienced crew, but you might see if you get seasick on a one day trip before you commit to a longer voyage.
Fill in the details with the info about where you would like to sail, how long you can go, and any other talents you might bring to a boat. A good cook, scuba diver, storyteller or engineer can be very valuable to a captain.
Also, look at the boat owner's profile. Findacrew has begun reviews and recommendations for both crew and captains. Seeing a positive review on David's page, from a woman, made a difference to me.
3. Finally, the most important part, if you ask me. Developing (if you haven't already) and listening to, your instinct and intuition. Do you trust your feelings? Have you ever made a decision based on them? Of course you have! If something feels bad you avoid it. That's trusting your feelings. When you rely on your instinct and intuition, you're looking at your feelings before anything bad can happen.
Let's say you meet someone new and something doesn't sit right with you. You don't know what it is, and it doesn't matter (in this case). It could be something big (like you feel so repulsed you think they might be a serial killer) or something small, like an incosistancy in something they said. If you were meeting someone face to face you might explore further to see what it is and get an explanation. "Oh" they say, "No, I meant to say I'm fifty-four, not twenty-four". And there it is, an explanation for your feeling.
But if you are just corresponding by email and phone at first, you have to be very sensitive to any red or yellow flag. If something seems wrong, and they can't or won't explain it, take it seriously.
Learn to be sensitive to your subtle and not so subtle feelings and reactions to people, places and things.
Just a few months ago I cancelled a sailing gig I was to go on based on an uneasy feeling. It was probably just that I wouldn't necessarily get along with everyone on board. But that's enough on a sailboat. On the crewing gigs that I have gone on, I have made peace with the risk factor, and the fact that I may have to walk away at any moment if it didn't feel right or safe. I am fortunate that I have had mainly very positive experiences so far.
I have also had a captain back out last minute on me. I was very disappointed, but he said that his wife didn't like my picture (?!). Both parties in this arrangement have to feel comfortable.
In the end you are responsible for yourself. Due diligence on the web (you can google boat and captains' names, look at recommendations) and in yourself (notice and value your own intuition) can go a long way. Pay attention to the information you gather. I hope you have great experiences out on the seas!