For women suffering from excess libido they can try chaste tree berry (also good for PMS or Menopause).  This has not shown to be effective for men.

In general hypertension, diabetes, alcohol, cigarettes, and prescription drugs all contribute to sexual dysfunction.  The worst of the prescription drugs are the antidepressants (prozac, zoloft, paxil, effexor).  For men there is a extremely high correlation between diabetes and/or smoking to their erectile dysfunction.   For women taking birth control with progesterone (which is quite common) and hormone replacement in general can also cause sexual problems.

For men natural cures include yohimbine (this can be risky) and soy lecithin.  General cures include gingko and chromium.

For women a popular lubricant recommended in "The People's Pharmacy Guide to Home and Herbal Remedies" is Corn Huskers lotion.