Having motivation is very important if you're making almost anything, from games to art. However, this is often very difficult to get, resulting in many developers abandoning their projects.
If you have any methods or just general advice on how to be motivated, post it here. We might eventually all learn something.
My advice: Take a break if you've done something boring or exhausting, such as school or workout.
Continuing your project immediately might just result in a lack of concentration (and/or awesome).
Also, watching anything sexeh is usually demotivating