How do I tell my girlfriend she needs to improve her oral sex?

Don't be an ass. Be specific, be open and be respectful.

How do I tell my girlfriend she needs to improve her oral sex?

As in any aspect of your life, the key to good, constructive feedback is specificity. Before you go dropping a bomb on your partner, be sure to figure out exactly what your complaint is: Is it frequency? Technique? Teeth?

The next rule: Be gentle and complimentary. Sexual skills and performance are sensitive issues. Make sure your girlfriend understands that you value her dedication to pleasing you (that she goes down on you in the first place) and that, all in all, you’re satisfied with your sex life. Also, let her know that you want to please her and are open to any suggestions or thoughts she has.

Say something like this: “I really love our sex life, and I think it’s important that we talk about it, check in with each other, and figure out how to make it the best it can be. I feel awkward talking about this, especially since I know I don’t do everything perfectly myself, and I’m sure you have feedback for me, too. But I want to talk to you about [insert oral sex problem here].” Follow that with something like, “I hope you know I think you’re amazingly sexy.” 

Give her the feedback—and some time to digest it—then check in with her about how she’s feeling about it: “Was it OK that I said that?” “Do you understand that in general I love our sex life?”

Then, fella, make sure you ask her if there’s anything you could do better.