You need to sit her down in a non sexual situation and have a heart to heart talk. Use "I" statements. "I feel..." "I would really love if..." Don't use "you" statements like "you don't have sex with me enough" or something like that. It'll just cause an argument. Tell her how sexy you think she is and how you'd love to be able to please her more often. Make sure you emphasize the "please her" part because otherwise it comes off like "I just wanna get laid more."
As with all things relationship-wise, good communication is key!