When you go to jam sessions, talk to others and ask if they'd be interested in playing with you. Post more than once on craigslist. Search Meetup for music and/or guitar meetups in your area.
Following are some NYC-area specific organizations that offer the opportunity for musicians to get together in more or less structured environments. They do cost some $, but they give you a chance to meet other developing musicians and play in ensembles, often with a teacher/leader.
For Jazz
- New York Jazz Academy (they have workshops on Long Island and in Queens)
- New York Jazz Workshop
- SOJ Jazz Center at the Collective
- Lucy Moses School (Search under Music->Jazz)
All the above offer workshops and ensembles - the first two especially have different ensembles you can try to get into.
Also(Also, if you think you're really good, many jazz clubs in the city have jam sessions, often late at night. The level tends to be quite high though.)
For Rock
- The Jam NYC (Jam sessions and networking - chance to form your own band)
- The Studio (Structured workshops and jam sessions, and probably the chance to network as well)