Im not necessarily a singer, but breath control is almost the best way to go. If you take correct breaths before the next like you sing, your voice will not only be stronger for that part, but after the night is over, you will still have your voice. Also, since you are singing in a choir, try to blend your voice with everyone elses so that you arent carrying the section. So make sure you arent too loud, but not too quiet. People often lose their voice because they are carrying a section, rather than blending with everyone else.
Also, warm up in the car or before you sing, before rehearsal, and before sunday morning, or whenever else you have to sing. So, try something like 1,2 3,4,5,6,7,8,9,8,7,6,5,4,3,2,1 You go up the scales through 9, and then back down. So if you play piano, in C its C D E F G A B C D C B A G F E D C. And modulate up...you dont have to start at C. I would start at B or something. So that will help your voice warm up properly, and even help increase your range.
Hope that helped