Try to practice mindfulness meditation. Mindfulness in Plain English [1] is a good resource. For starters it is relaxing in itself, and will teach you to relax faster, but if you practice with eyes clothes in a chair or on a couch instead of sitting in a traditional pose, and you're actually in need of sleep, it's easy to drift off unless you actively try not to.
The book has a number of pieces of advice to avoid sleeping - if your aim is to nap rather than meditate, just do the opposite. E.g. part of the point of a traditional meditation pose is to create alertness (sit up straight, lift your head etc.) in part to avoid falling asleep.
Sometimes I want to stay awake and pay attention to pose etc. Sometimes I like to use the breathing etc. to have a quick nap, and instead will sit in a relaxed position, rest my head. I find it's made a big difference in how quickly I fall asleep - I now associate certain positions with "fall asleep ASAP" and certain positions with "clear your mind and stay focused", and automatically fall into certain patterns when I prepare accordingly.
The book has a number of pieces of advice to avoid sleeping - if your aim is to nap rather than meditate, just do the opposite. E.g. part of the point of a traditional meditation pose is to create alertness (sit up straight, lift your head etc.) in part to avoid falling asleep.
Sometimes I want to stay awake and pay attention to pose etc. Sometimes I like to use the breathing etc. to have a quick nap, and instead will sit in a relaxed position, rest my head. I find it's made a big difference in how quickly I fall asleep - I now associate certain positions with "fall asleep ASAP" and certain positions with "clear your mind and stay focused", and automatically fall into certain patterns when I prepare accordingly.
[1] http://www.urbandharma.org/udharma4/mpe.html