Keep playing it over and over again until you become very familiar with the piece. Make sure, however, that you are counting correctly while playing. Try not to rush, and keep a steady, even pace. Keep calm, and focus on the piece, and don't get distracted.
Practicing tips for the future: even if you do not have a lot of time, a few minutes of quality playing is much better than a lot of incorrect playing, such as poor mouth or finger position, or playing wrong notes or rhythms. Make sure that you are good on the rhythms and notes, tempo, dynamics, and any other specific details that are important.
Overall, proper practice and a steady tempo are very important.