When choosing songs, stick to material that falls into the kind of 'core' blues repertoire your fellow jammers will be most comfortable with. A jam set is not the place to break in new, obscure or unusual numbers. Likewise, don't expect anyone to be able to read sheet music or even charts.
Vocalists - as set-leader, it's your responsibility to call the songs clearly so that everyone else onstage knows what kind of number to expect. If it's a well-known blues standard, by all means give the title, but don't assume that everyone will know how it goes. Describe it. "medium shuffle in G, quick four, watch me for the stops" - that's the kind of information your fellow musicians need.
Whatever the song, it doesn't matter how many verses or solos the original recording had: you have to ensure that all of your fellow jammers get a chance to play a solo if they want one. When handing out solos, make it very clear who it's being offered to, and make sure they've got the message. Big, definite gestures are better than a subtle nod or a vague waft of your hand.
IMPORTANT NOTE ABOUT SOLO LENGTHS: Time-wise, soloists are always given "twice round the block" - i.e. two complete 12-bar progressions. Any shorter than that and they won't have time to develop what they're doing, and both the musician and the audience will feel cheated. Don't forget to throw a solo to the drummer as well!