Hi, Aida. I'm a voice teacher and vocal major at a University in the U.S. The main thing to remember when singing over a period of time is not to exhert your voice when you're not performing. Some people call this "vocal rest". Always keep your throat lubricated. I always suggest drinking lots of milk (chocolate milk does the same thing but tastes nicer!); this will coat your throat and keep it wet. It's important not to drink the milk any closer than about 2 hours before you perform, however, because it will darken your tone when you sing; switch to water instead. If you find that your voice begins to hurt when you sing in rehearsals, back off! The only thing that really matters is the hour or two of the performance. I also recommend "warming up" to about a third or fourth above the highest note your singing, and at least a fifth under your lowest. This will make your performance feel easier. Hope this helps.