L.L.
First, use a good spray/foaming spray, leave it on for twenty minutes...and not just a light spray. My mom taught me this (and I swear, I always wondered how her showers were spotless, year after year.) She pretty much uses half to a whole bottle each time. At least half a bottle...she really coats it.
So, come back in 20 minutes, and rinse and scrub a bit. Then sprinkle it with Comet...a lot of comet. Then let that sit for just a minute or two, and really scrub it good with a scrubby sponge.
Final step, spray it with Windex and wipe it down and it will really shine!
Now...when I say use half a bottle, I'm talking about a tub with all the walls. If you're just cleaning the bottom of the shower, it won't take nearly as much...but really cover every inch of it.