Write the letter in handwriting that's different from your own. If you usually write in cursive, then print your love letter. You can't stay anonymous if your crush recognizes the handwriting on the letter.
Use phrases in your anonymous love letter that you don't normally use when you speak to your crush. He may guess who sent him the love letter if you write a phrase that you often use while speaking to him. Most people say things out of habit, so think carefully when choosing your words.
Spray a bit of perfume or cologne on your love letter to give it an extra romantic touch. It can drive her wild knowing what you smell like but not what you look like. Use a fragrance that you don't wear every day. When the time comes for you to reveal your feelings, wear that fragrance and they should get the hint immediately.
Have a mutual friend deliver your anonymous love letter. You must be sure that your friend keeps your identity a secret, though. If your friend isn't trustworthy, mail the anonymous love letter instead--but don't put your return address on the envelope.