There are three provisional cast-ons that I regularly use, but the one I'm using today is the crochet cast-on--not the one where you make a crochet chain in waste yarn and then knit into the bumps at the back, but the one where you make stitches around a needle with a crochet hook. With my current project, I want to able to pull the waste yarn out easily and put all my starting stitches back onto a needle for working in the opposite direction. The beauty of the crochet cast-on is that the waste yarn pretty much "unzips" itself. No picking out bits of contrast colour stitch by stitch.
Here's how to do it. First, you need a needle in the size needed to get gauge (I'm using a straight needle here), a crochet hook about two sizes smaller than the needle you'll be knitting with, and waste yarn of about the same thickness as your working yarn.
Start by making a slip knot around your crochet hook.
Next, holding the needle to the left of the hook, take the yarn behind the needle,
and then around to the front before hooking it through the initial loop on the hook. Like this:
Now, take the yarn between the hook and needle and then back of the needle before making the next stitch in the same way.
If you're doing this with waste yarn as a provisional cast on, then simply break off the waste yarn and begin knitting (onto a circular) with the working yarn, like this:
This is a really useful cast-on to know, because it has a second use, which is as a cast-on that mimics a bind-off. If you want the start and end of your knitting to match, this is the cast-on to use. Simply cast on in the yarn you intend to continue with, instead of with waste yarn. FYI, I used this method to cast on for this scarf. It was knitted horizontally and it mattered that both edges matched.
Be aware that when worked as the lower edge of a piece, as I used it in James' scarf, the cast-on edge will develop a nice amount of stretch when released from the straight needle, another factor which made this method so desirable for this project.
P.S. (posted Mar.21/14)--For guidance in removing the waste yarn and getting all those stitches back onto a needle, see here.