"Frequent sex is linked to better memory", seems to be the suggestion from a number of recent studies
A. A 2010 study on male rats indicated that those having sex daily for two weeks had more neuron growth, and enhanced cognitive functioning, compared to those who had sex only once in the period.
B. Then came the question of applicability of humans, which seems to have been answered well in a study published in the Archives of Sexual Behavior. It seems that there is indeed applicability!
It must be noted that these are co-relational studies (which means that cause-and-effect debate can still rage on, but then again, why take chances? Just have more sex, eh?
And in case you are wondering about your overall sex life--problems, frequency and such--here's a handy quiz that you can complete in four minutes!