1/4 cup ammonia (not sudsy)
1/2 tsp liquid dish soap
5-10 drops of lemon oil (100%pure concentrate, I got mine at a health food store--this cuts the ammonia smell some)
Fill the rest of the way with water (this is for a 32 oz bottle)
Spray on shower doors and walls after showering, you don't need to rinse. Super cheap and so far (2 weeks), it is working just fine.