For a monogamous relationship in which only one partne has herpes, condoms should certainly be used while the disease is active. This risk of contagion is low at other times, but eventually the other partner will probably become infected. However, if the couple has already had unprotected intercourse for 1 or 2 years, there is a good chance that the other partner is also infected with herpes and doesn't know it. Many or even most people with herpes never have symptoms. Nonoxynol-9 seems to work in the test tube, but there is no proof that it works in real life.
Condoms probably work better.
Diaphragms won't help very much at all. Genital herpes can be spread by direct contact to any part of the body, including the mouth, finger, legs, abdomen, and rectum. As long as there is no direct contact with the infected area, other types of sex are probably safe. In terms of its physical effects on the body, it is not a serious illness. It causes much less discomfort than migraine headache, asthma, and arthritis, for example.