The best way to get better at something is to do it. :)
Play a game with yourself. Think of a concept/word, and explain it (out loud) as best you can with the vocabulary you know (kinda like Catch Phrase). Circumlocute. Alternatively, if you're in a social setting, with others who know your target language, you could actually play Catch Phrase.
Of course, solo Catch Phrase can get a little boring after a while. Here are a couple more ways to spice up your studying:
- Try describing what the word/concept is not.
- Add a time limit (the game Catch Phrase already has one).
Of course, regular old vocabulary study is going to help a bunch here, too.
Studying alone is not necessarily going to get you to where you need to be, speaking-wise. Converse with L2 speakers as much as possible. If you can become good friends with one, you'll feel more comfortable speaking with them, and you won't be as nervous about stumbling.
Keep practicing!