DO practice common phrases and useful words right before you go, so that they are fresh in your mind. You'll feel more confident if you don't have to stop and think everytime you want to say something.
DO carry a phrasebook or pocket dictionary (or if you travel with your phone, think about downloading a dictionary or translation app). Everybody has those moments, no matter how good your English is, where they just don't know a word. Having a tool like a dictionary is perfect for those moments.
DO talk to new people. I know, all you extroverts are thinking that's an obvious point. A given. Why even put it in the article? But for all you introverts, listen to me. Talk to people. You don't have to have long or profound conversations or become best friends, I promise. Go ahead, strike up a conversation at the tourist site. Talk to the people sitting next to you on the plane or at the park.
DO have fun! Using your English in a real-world situation can be challenging, but it should also be rewarding. This is your opportunity to experience a different culture, meet new friends, and expand your horizons!
DON'T be embarassed about your English! Honestly, half the people you speak to will be impressed that you've learned another language at all, because they've never done it. And the people who have studied another language will understand exactly how you're feeling and help you as much as they can. Go ahead and tell a native English speaker that your English is not that great; I can almost guarantee that they'll say something like, "No, you're doing great! You speak English better than I speak your language!"
DON'T get offended when people correct you. This is your chance to learn and improve your English! Also, the more polite native English speakers are to you, the less they care about what you're saying. So if they're correcting you, it means they're really listening.
DON'T worry too much about getting everything "perfect". Nobody speaks perfectly, that's one of the major differences between spoken and written languages. Sure, take a moment or two to get your thoughts in order and think about what you want to say... But then go ahead and say it! Don't be afraid of talking just because you can't think of the precise word or verb tense you should use at the moment.