Work on your preparation until you can do so while remaining relaxed. If you're too tense during preparation, the resulting pirouette will likely be out of control.
Add more abdominal exercises to your training regimen so that you become familiar with engaging your core. Your core muscles are absolutely necessary for staying balanced so the stronger you are in that area, the better.
Push off from your plie just enough so that you make it all the way around once. You might be surprised at how little energy you need to dedicate to the push. Once you master the single turn, work you way up to doubles and triples using the same 'just enough' philosophy.
Master your spotting technique by doing a couple practice spins before you practice your pirouettes. Smiling as you do this will help you find yourself in the mirror and stay relaxed as you turn. Better spotting will help improve your timing and balance overall.
Step out of the pirouette when the time is right. Many dancers stay in their pirouette turn until they stumble or fall out of it. You can improve your turns by focusing on your balance from start to end so that you finish gracefully and in control.
Practice pirouettes only when you are in the mood to do them properly. If you're pirouette is less than perfect, you may form a habit of doing them improperly and without the correct technique. They only way to break this habit is to commit to doing them the right way every time. This doesn't mean that you won't make a mistake. Rather, this means that you'll maintain focus and pay attention to every detail of the pirouettes you practice until you can nail them consistently.