I'm sure some of this advice will be repeating the above, but:
Go early. Use early entry, particularly for Magic Kingdom and (if you have any interest in Tower of Terror or Rock N' Roller Coaster) MGM. If not using EE, go at park opening time to one of the non-EE parks.
Read in advance how to use Fastpass and take advantage of it!
To the extent it makes sense with EE and such, take advantage of your location by going to MK while you are at the Poly---you can't beat the easy access by monorail or boat launch. Go to AK while you are AKL. It's also an easy walk from Poly to TTC to take the monorail to Epcot.
Check the park schedules in advance. If Spectromagic parade is available on a weeknight, see it then, so you can avoid MK on Saturday. Take advantage of park hopping---you can spend the morning at AK (which closes relatively early), take a mid-day break and then hit a late-closing park.
Don't overplan your meals way in advance. Make a PS for a nice dinner at a resort (not in the parks) on your waterpark day (CA Grill, Flying Fish, Artist Pointe). Otherwise, try to stay flexible and don't plan on too many full service sit-down meals, which consume park time and force you to be somewhere at a particular time. Consider eating your sit-down meal as an early or late lunch, giving you a mid-day break and avoiding some of the crowds. At the end of the day, decide what you are doing the next day and if you want to make/change/keep your PS arrangements. (But (1) probably want to make a PS for Saturday night, and (2) don't miss Boma at AKL).
While you could spend weeks at Disney World and never see it all, you'll easily get to all four parks within the 6 days you've got, and should see a lot. Don't get too "commando" about seeing "everything" and miss out on what makes Disney special--live entertainment opportunities, little surprises around every corner, astounding detail, beautiful landscaping, etc.