Don't ask for a date in the first letter. But don't wait fot too long. Ask her out when she's ready.

DON’T offer her an off-line date in the first letter. Chances for success are slim. Most girls will turn you down. Instead make her interested in you. Start a conversation.
And on another hand…

DON’T’t make your correspondence too long. There is no universal rule on the correspondence length. Sometimes you can ask her out in a second letter, sometimes even 10 is not enough. Just try to feel the right time to ask her out. But don’t wait for ever. Girl like resolute men.

Watch for girlfriend. With some reason many girls decide to go on-line along with their girlfriends. Make sure you never write to both girls. Even if you come up with two different letters, still don’t send them to both. Girls want to be unique. Make her to believe that you don’t write to anyone but her.
I new a girl who posted her profile on a dating site together with her girlfriend. And they immediately ruled out guys who wrote to them both. No matter how good these guys were. Here are some tips for detecting girlfriends:
they appear on the same day.
They may have similar pictures / profiles
they use the same mail service
If you suspect that these girls are together – pick the one you like most and write only to her. It’s better to get one answer then none.