Recently via another blog (sadly I do not remember which one) I came across a method that Meg Swansen published in a Winter 2008 Schoolhouse Press Newsletter. This method involves knitting into the front of the stitch and then bringing the needle to the back and slipping the original stitch. Full details are in the linked newsletter article, about halfway down the page.
Another method is to actually knit into the back of the stitch first and then into the front (kbf). This does twist the base stitch but it also avoids the purl bump created by the kfb.