How can i improve my singing?

im not bad at singing, its just that when i sing a lot, mucus builds up in my throat. also, sometimes it sounds like im lisping my s's & its hard to really get my diaphragm into it. i have really good pitch and tone, i just need more "mmph" into my voice if you know what i mean. i want to improve my voice for my school's play next year. help? :)