As everyone has already noted, it is not easy to get from one resort to another via Disney transportation. Your best way is to use the nearest theme park for the day (or part of day) that you plan on going to another resort. You can also boat to the Wilderness Lodge and the campgrounds.
From the MK you can easily get to the Contemporary (walk or monorail), the Poly (monorail or boat) and the Grand Floridian (also monorail or boat).
From the Epcot International Gateway it is an easy walk to the Beach Club, Yacht Club, Swan and Dolphin. But note to exit at the International Gateway you would need a park admission.
You can also reach the Epcot resorts from the Studios without using an admission. Just take your bus to the Studios and then either walk or take the Friendship boats to the Epcot resorts. This boat also continues on to the Epcot International Gateway.
If you plan on visiting other resorts late the easiest way back to your resort is by taking a taxi. The bus to DTD and then to your resort takes a LONG time. Unless there is a theme park open these would be your only two options late at night.