Start off with the accurate address of your friend and almost needless to say, do not mess up with the spellings of his/her name (unless you want a smaller piece of the cake)
Greet the friend with all the love in the world, after all it is going to be his/her day very soon.
Of course you would know the age of your friend so do not forget mentioning the number and your happiness over it. Continue with the fact that you waited for it the entire year and are glad that finally the day is nearby.
Wish your friend all the luck and prosperity for the coming year and for the party too. Make sure that you mention about such invites flowing in your way each year.
There is no harm in reminding your friend the way you people celebrated the party last year. In case you remember the things which went wrong and want them to be spot on this time, then toss in a reminder through the acceptance letter.
Do not forget to assure your friend that you will be right there for any help that might be required. Tell the friend that you will be available even before the birthday if the need arises.
In case you are a very close friend then you can even suggest revision in the guest list.
Close the letter on a thanking and looking forward note.