If you're a beginner you need stage time, and you need to watch improv. You have to get comfortable out there, and that comes from flight time. Take every opportunity to play. Watch the more experienced actors, see how they react to tough situations. Note how they play and when they follow "the rules" or break "the rules" and how they do it. Be a sponge! Also meet people, get familiar with the scene. The best way into joining or forming a troupe is to play with people.
If you're a journeyman you need stage time, and you need to watch improv. Jams are perfect for stretching yourself and trying new techniques. You'll hopefully get equal measures of playing with more and less experienced people. When with newbies, you'll exercise support in difficult situations, dealing with denial (god bless them) and making your scene partner look great. With more experienced players you'll hopefully get the support to play more adventurously than you would normally, or get pushed into doing something you've never had the opportunity to before.
If you're an established player you need stage time, and you need to watch improv. It's really easy to get into bubbles when you're more established, playing with the same people or the same types of forms over and over again. Jams will shake you the hell up, force you off balance and out of your comfort zone. Playing with a bunch of people you'd never normally share the stage with can help break you out of your go-to rhythms. Plus, watching new people play is fascinating, and often inspiring. Nothing is better at cracking up your calcified notions of improv than seeing a new person rock a scene doing things you never would have considered. Nothing energizes like seeing developing talent.
And, for everyone, go to jams because they are fun and support the community.
JUST GO.