Subscribe to "Writer's Digest" magazine. You'll find tons of great workshops, including specialty workshops for screenwriters. You'll also get valuable writing advice on how to improve writing techniques as well.
Develop a basic outline for the screenplay you're currently working on. You need to have something to work with at the workshop, and this will give you something to practice the new character and plot development skills you will learn.
Find screenwriting workshops at MovieBytes (see Resources below). You can search for pertinent contests and workshops to gain exposure and hone your screenwriting skills. They also have user ratings so you know which opportunities have provided the best experience to their users.
Consider purchasing specialized software such as Movie Magic's "Screenwriter" program. This is specialized software with tools created especially for those writing screenplays.
Share your experience with your writer's group. Be sure to let your writer's group know that you're considering attending a screenwriters workshop. You'll gain valuable insight into what others have experienced and they may help you hone your screenplay for the workshop or give helpful advice of what they found most useful when they attended a screenwriters workshop.