If you are trying to let go or forget someone...
Don't talk with him on the phone, chat with him online, send him an SMS or see him for 21 days.
Don't look at his photos. Don't read his letters.
Keep yourself busy with other interests.
For 3 days, you will miss him terribly. Pain will manifest itself physically.
After 21 days, you will feel less attached. You may even start to forget the sound of his voice or the feel of his touch.
If you are trying to sleep early...
Get yourself really good bedsheets.
Don't watch TV atleast 2 Hours before bedtime.
Put your Ipad away.
You can take over the counter sleeping aids for 3 days, only if you have to.
Close your eyes and just accept that it's time to sleep.
After 21 days, you will just naturally feel sleepy.
If you are trying to lose weight...
Don't give up everything you love on the first day.
Try to substitute rice with lettuce or reduce it to half a cup.
Try to walk for 20 mins every day.
The first 3 days is always the hardest.
By the 4th day, you will feel that your body wants to do more.
After 21 days, you will crave to exercise and you will not crave for your usual favorites.
It only takes 21 days.
PS> I have a project to exercise for 21 days. Today is my day 1. I ran 5km for 39:11 minutes.