Understanding what motivates people goes a long way to figuring out how to keep them motivated. If you want to keep a sports team motivated, then they need to have a clearly defined goal. For an Adult Student, the key to staying motivated, is having a clearly defined goal, and working to achieve it. Without a specific goal to achieve, there is nothing to keep a person motivated. One downfall with setting goals and thus causing people to loose their motivation, is that people set unrealistic or unspecific goals.
An example of an unrealistic goal, may be "To test out of all of my English classes to eliminate 4 semesters of classes, work full time and graduate with a 4.0" That is very hypothetical, but it would put a huge amount of pressure on anyone to do that.
Now, ask yourself, and answer honestly, "Am I willing to do whatever it takes to see this goal through to the end?"
Try making a bunch of short term goals, rather than long term goals that you can loose your focus on.
Whether you celebrate with your loved ones, or have a drink with friends, buy yourself something you have long admired or spend the day playing with your kids make sure that you acknowledge at least to yourself if no one else that you made it to your goal.
To keep motivated, make sure that you always have a goal, whether it is for education, or for anything you choose to do in life, and make sure that you are rewarding yourself for each step along the way that you achieve completion of that goal.