1 cup vinegar
1 cup Dawn dish soap
Mix together and pour into a spray bottle. Apply to shower or tub. Let it sit for a few minutes (this lets the vinegar smell dissipate a little). Wipe and rise. Be prepared to be amazed.
Update: I have been using this for over a month now and it has become my #1 choice in cleaning products. I use this to clean everything in my bathroom, even the toilet. My toilet sparkles like never before. I use it in the kitchen to clean my stove, microwave, the cabinets above the stove which seem to always get grease up there and tons more. I promise you will love this stuff. It truly is amazing.