Whatever you do, don't use WD40!! It will attract water and make the noise worse. If I'm not mistaken, one of the better ways to do this is to apply a little grease on each side of the bushings.
There are four bushings with two sides a piece, so 8 tiny squirts of grease should do the trick. In so doing, the small amount of grease will work its way into the bushing (with general use of your truck) and hopefully eliminate all noise. For best results, apply grease again after a week or so to make sure it's in there nice and good.