Hi! My great-aunt turned me on to keeping a squeegee in the shower so that after every shower I can get the water off the glass, and that helps greatly. To get them clean to start with, though, get a foaming bathroom cleanser and spray a thick even coat on, and let it sit there for a few minutes. Scrub it off with a sponge and you should have lovely clean doors. Windex the outside to get rid of any regular water streaks, and then squeegee every time, and you should be good to go.
I've also started a new thing (this morning, in fact) trying to keep ahead of 'soap grunge.' I bought a pretty shower-poof thing at Wal-Mart, and after my shower this morning, I poured some Clean Shower on it and scrubbed the doors and rinsed them and then squeegeed. And the shower poof just stays in there all the time with my squeegee.
(Are you familiar with FlyLady? This was based on her tip that, if you quickly clean just one wall of your shower or tub every morning, the thing never really gets dirty, and it takes you all of 20 seconds. flylady.net for more)
Hope this helps!
J.