"Each theme park has direct buses to the other parks, so you don't have to go back to your resort."
We did not find this to always be the case when we were there in December. Rarely did we see a park to park bus at a park.
If you don't see a park to park bus the easiest way we feel is to get on a bus to a resort closest to one of the parks - either the one you are coming from or the one you are going to and then get on the bus to the park you want to go to. For example, if you are going to EPCOT and have been at AK either take a bus to Animal Kingdom Lodge or a bus to the Beach Club and then catch the EPCOT bus from AKL or the boat from Beach Club (or it's a very easy and nice walk from Beach Club).
Sometimes you can catch a park to park bus at the Ticket and Transportation Center but sometimes it's hard. Also allow an hour for the transportation, just to be safe.