Some good sites on the net that give the procedure for Avani Avittam and Gayathri Japam (helpful especially for those who plan to do the same by themselves):

1) http://www.trsiyengar.com/id24.html

2) http://www.ahobilam.com/

3) http://www.kandamangalam.com/Avani%20Avittam%20Audio.aspx

4) http://www.prapatti.com