When?

Any Time:  Tertiary erogenous zones, ie the skin, can be touched at any time and may or may not produce a sexual effect.  Secondary and primary zones are much more easily stimulated and should only be touched in a sexual context, when you want to set the ball rolling.

During Foreplay:  The skin (tertiary erogenous zone) can be touched and stroked, as well as secondary erogenous zones closer to the genitals.  Play with the nipples or breasts as well, but it’s best to leave the genitals themselves until you are both more stimulated for maximum effect.  Touching too early on can sometimes be uncomfortable and be interpreted as an assault on your partner’s personal space.

During Sex, when you are both highly stimulated.  All erogenous zones can be touched at this point, but the primary zones, like the penis, vagina and clitoris can produce orgasm, while secondary zones will increase pleasure.  Touch and stroke the skin for a variety of sensations.

After Orgasm:  It’s best to avoid touching the penis and clitoris directly after orgasm, as they need a little time to recover after the intensity of the orgasm.  They are very sensitive at this point and may be uncomfortable or even painful to the touch.