Start slowly.  You don't need to be a professional writer to create a decent love letter.  You can talk in a letter, get to know someone in a different way than just over a computer.  It can gradually grow into shared intimacy and trust.

If you need inspiration try looking for it at your favourite art, literature and/or music for inspiration.  I bet some of your favourite songs are good examples of expression of love.

Place a picture of your loved on in front of you for added inspiration.

It is OK to practice on plain white paper.  When you have written what you want to convey to your love, take out some nice stationery and write the final copy on that.

Do not try to gain points by writing phoney letters about such things as flowers, butterflies or something else that your partner may like.  A love letter should state what you really feel not what you think he or she may want to hear.  Write AT LEAST  three qualities about him or her that you love the most.

Do not write about anyone else except you and the addressee in the letter.

Always end the letter with thoughts and hopes for the future.

Love letters need not only be for new couples.  It can be a great way to rekindle the romance in a marriage when the honeymoon has been replaced by comfortable security.