Taken from www.SexualStress.com - this website has lots of great articles on sexual issues.

When a couple's sex life is not as they would like it to be, it is seldom the entire fault of one or the other - though it can often feel like that. If an individual is unhappy with their sex life, an important question to ask themselves is: "What is my partner's sex life like?" Where this question cannot be answered, that is frequently an indicator of the nature of the problem and illustrates the need to be able to communicate clearly. If this sounds like your problem, the following 'rules' might help you:

Ask if your partner is willing to discuss a sexual matter with you. If they say "No," you can go no further and you must find a different way to work.

Ask if they will allow you to finish what you want to say.
Be specific. Using euphemisms and metaphors often does not allow a proper understanding or resolution. Use the proper words for parts of your or your partner's body, where possible.
Be sure to say clearly all that you need to say and remember that keeping it brief usually aids clarity.
Don't be shy or coy. This can often make matters worse. Act confident.
Discuss your partner's responses calmly, even if they are manifesting anger or ridicule. Ask, without anger: "Why do you want to [hurt/ridicule] me?" if that is the response you get.
Remember that animosity of any sort only tells you how your partner feels. Listen and calmly discuss but don't necessarily believe that what they are saying is justified.
Don't expect miracles - and if you are male, don't expect sex immediately!