I know it sounds cliche, but just keep waiting! I waited for at least 4 years before I got my Placido Domingo autograph - and I lost track of how many letters I sent him.
I've had dry spells that have lasted over 3 months, and when you get something in the mail it's fantastic.
If you want authentic autographs, check in the "Is My Autograph Authentic" forum and see who sends out PP, AP and Secretarials.. and choose not to send to them.
Another tip - send only what you can afford (monetarily or otherwise) afford to lose.
Remember, this is a hobby - it should be fun. When it stops being fun, then stop.
Personally, I've sent out more in 2009 then I did in all of 2008, and I haven't gotten many successes yet, but they should come in soon.
Try to write to other stars that you like, not just the ones that are in the spotlight right now - you might have better success.
Favorite Autographs: Luciano Pavarotti (IP), Joan Baez, Dorothy Hammill (IP)
Newest Autos: Tim Allen, Daniel Radcliffe x3, John Lithgow
Visit My Site (not all autos up yet!):
http://selise.fotopic.net/ <-recently updated