Falling asleep should be easy and natural.
If you find yourself in bed trying to fall asleep for more than 15-20 minutes, then it might be a better idea to get up. Do something quiet and relaxing but not stimulating, such as reading a book, tidy up your desk or listen to some relaxing music. It will help you relax, unwind and eventually feel sleepy.
Perhaps this is something I should try to keep in mind and practice.