I agree with Ware Bears, if possible - definately try to visit on a weekday.
Personally, if I had a one-day visit to DLP - pressuming you have a ticket to access both the Magic Kingdom Park AND Walt Disney Studios Park. I would start my day in Disney Studios - being at the gates for the moment it opens. I would rush onto Tower of Terror, then Rock and Roller Coaster and then Crush. You then have done the big main-rides in that park. If you want to stroll around and visit the other attractions - that's at your leisure. This should take you upto around midday.
I would then head out and over to Magic Kingdom and bee-line to Space Mountain : Mission 2 in Discoveryland, from then follow the lands around taking some photos along the way (Fantasyland is pretty beautiful with lots of hidden gems) - whilst in Adventureland I would then head to Indiana Jones, then do Pirates of the Caribbean - finally in Frontierland I would either queue for Big Thunder Mountain or grab a fastpass (depending on the queue) and head for Phantom Manor.
A tiresome day but one that would be well-worth it! Do as much as you can. If you get a chance to see a bit of the parade then go for it - but if you really aren't too fussy about catching the Parade - USE THIS TIME WISELY!! Whilst the afternoon parade is going on in Magic Kingdom the rides are pretty much EMPTY - so go for the big ones around this time (Space Mountain, Big Thunder Mountain, Indiana Jones, Buzz Lightyear - all the ones that get busy).
Finally, once your loooong day is over and the park is closed - treat yourself in Disney Village to some food - a few drinks and some shopping... Most of the merchandise in the Parks you can buy along Disney Village anyways - so why waste an hour of valuable park-time on shopping when you can do that once it all closes?
Have fun - and wear comfy shoes!!!!