Your body is already doing everything it needs to prep. You may notice during pregnancy that the areola around your nipple becomes a bit darker, and sometimes the nipple itself seems to change in texture. These changes are hormonal responses to your pregnancy and help to prepare your nipples for feeding.
You may notice some products on the market that claim to prep nipples, but there is no need to use any of them and no advantages to doing so.
In the past, mothers were encouraged to "toughen up" their nipples before birth by rubbing them with towels or a loofah (ouch). The good news is that this is absolutely unnecessary. In fact, your nipples are supposed to be soft and comfortable in baby's mouth — not "toughened up."