Always remember that eating a girl out is all about pleasuring her and not yourself. There is no bigger turn off than a guy who starts licking and then tries to position himself so that he can get some pleasure. Giving proper oral sex is about taking your time and gently exploring every crease and fold of your woman's vagina with your tongue. If you take this unselfish path then she will get so much more out of it and will want to thank you for it. Instead of having her lay on her back with her legs open, try different cunnilingus positions when eating a girl out. A really good one is to have your woman kneel up and put her hands against the wall while you are on your back licking her from underneath. Just experiment with some different positions so that oral doesn't become boring.