Just found this in another thread -
While there may be no medical evidence to connect grapefruit juice or any of its properties with a thinning of the cervical mucus, women for generations will swear on the remedy. Because of this, it is definitely worth a try. You may have to drink the juice regularly for a couple cycles before you see a change, so hopefully you like grapefruit juice. If you want to increase your chances of success with this remedy you may want to combine your juice with Evening Primrose oil or even Robitussin, which are also known to thin the cervical mucus and create a more fertile environment for the sperm and ultimately fertilization.
Even if it is not the grapefruit juice that works its magic on your cervical mucus, this remedy will help because it will increase your fluid intake. If you have very dry or spongy cervical fluid you need to increase your fluids and grapefruit juice certainly will count! So, whether this generations old home remedy really does work or not is not really up for debate because it does work at least by putting more fluids into your body, and that is important. If you see increased results, than that is even better. Remember that there are many natural remedies that you can try and even combine for proven results. Generations of women and doctors cannot be wrong, so dont hesitate to try this remedy and see what happens. Even if it does not prove to be useful for you, you wont be any worse off than you are now."
I hadn't thought about upping my fluid intake - my DH calls me a camel as I can go all day without a drink (well I think that's why!) I'll try that if I don't need to drink grapefruit juice!