Let's dive right in with the best-kept secret I've discovered about shaving your bikini area: Deodorant.
No, seriously! Apply deodorant to your bikini area immediately after shaving and drying, let the deodorant dry for a minute if it's a gel (which I actually prefer; I feel it helps with my sensitive skin in a dry climate), then go on your merry way.
When I first learned this trick from some women on a mommy-to-be discussion board, I thought it was a practical joke. In my mind, there were several bored, nauseated pregnant ladies laughing themselves silly about the idea of the rest of us using deodorant to prevent razor burn "down there".
But I have really sensitive skin, and I was ready to try almost anything. This seemed simple enough, and nobody would have to find out I'd been punked if it turned out poorly.
So one afternoon I shaved and then pulled out my husband's stick of deodorant (the women swore men's deodorant works better) and applied it to the shaven area. I felt thoroughly foolish, yet cautiously optimistic.
To my surprise, it worked! Previously, I had been unable to shave my bikini line without enduring days of painful razor burn and ingrown hairs. Now I don't deal with the pain, and though I still get a few ingrown hairs, it's less than 5% of the ones I would get previously. And the few ingrown hairs I do have to deal with now don't hurt! I call that a huge success.
In addition to applying deodorant, make sure you shave properly. You want to shave in the direction of hair growth in sensitive areas, not against the grain. If you're really sensitive, make sure you replace your razor blades frequently and use a skin-friendly blade. This is my favorite so far with these blades if you're really sensitive.
Now, I would advise that you use a different stick than you do for your under arms, for hygiene reasons. Be sure to also clean it after each use, generally with a little pass from a towel or some toilet paper. Nobody wants to trade razor burn for a yeast infection!
If you're going to use a gel deodorant, some people recommend you let it dry a little longer before putting on your swimsuit; the deodorant could leave little marks otherwise. I can't be bothered to wait very long, but I do let it dry for a few moments so that it can be absorbed longer before my clothes start trying to rub it off.
Now, go forth and enjoy your summer with a trim and carefree bikini line!