Recently Summer Glau told fans at 2012 Emerald City Comicon video panel that she has a new fan mail address.
UPDATE April 2017: Here's the address you should use from now on if you want to request an autograph or simply send a letter to Summer.
17503 LA CANTERA PKWY
STE 104 #432
San Antonio ,TX 78257
The address Summer mentioned during her panel is no longer available.
3940 laurel canyon blvd.
Studio City, CA 91604
She was really excited about this because she is now in full control of her fanmail box.
Here are the whole process "How to send a fanletter to Summer Glau?" explained.
If you just want to mail her a letter and do not want anything back:
Put your letter folded inside a envelope, seal the envelope. In the front of the envelope in the middle you write the address (you don't need to write the country if you live in USA). Put some stamps on the envelope (in the top right corner, but not too close to the edge). Note that the cost varies according to the weight of the letter, so if you're not sure about how many stamps to use, then you should ask at the post office where they can weight your letter. Find a mail box and drop the letter into the box, done!
If you want to request an autograph and live in the USA:
It is very unpolite to ask the receiver for a reply if you don't pay for the stamps so send a properly stamped and self addressed envelope with your request letter and a photo. You can include a piece of cardboard to keep the photo from bending in shipping and also add "Do Not Bend" on the envelopes. Send your letter and wait.
If you want to request an autograph and don't live in the USA:
The problem is that it might be hard to find American stamps in your country, but don't panic there is a solution and it is called International Reply Coupons (IRC).
An IRC is a coupon that looks the same in all countries. The receiver can use the coupon to buy stamps. One can buy IRC at the local postal office. Add several coupons and place them into the envelope, not on it.
If you want to request an autograph and live in some exotic country that don't sell IRC or have post offices:
Visit the online webshop for a postal office in a neighboring country and order some IRC e.g if you live in Sweden you can buy IRC at the Finnish or Danish post.
For advice on what to write in the letter use this thread: Fan letter to Summer Glau