Choose an appropriate workshop. Read course descriptions and talk to or email current and former students as well as the workshop instructor. In order to improve your writing style and technique, you'll need to feel comfortable with the other participants and feel confident in the teacher's ability and experience.
Be pro-active. You'll need to read as much of your work as time allows, and offer critiques for the other students' work. The more you read and critique, the more you'll learn about vocabulary, dialog, syntax, plot, character and point of view. After you've taken a few classes, you'll get into the rhythm of proper reading and critique.
Take everything with a grain of salt. Most writing workshops have between 10 and 15 students. Most of them will have slightly differing views on your work. Take notes on their critiques, think about what they've said, but don't take it as gospel. If you get too many differing opinions, ignore them. If most of your fellow students find the same problems, consider revising that part of the manuscript.
Discuss technique with the class. Your teacher and other students will have their own writing styles, habits and favorite authors. Listen to their experiences and see if you can apply some of their technique to strengthen your own work. For example, you might use third person narrative if you normally use first person point of view.
Write as much as possible, in and out of class. Even if you only have a few minutes during your lunch hour, pen a quick paragraph. The more you write, regardless of genre or style, the more your technique will improve.
List problem areas specific to each story before presenting them to the class. This will enable the teacher and class members to listen more attentively to parts of the story you wish to improve.