Somehow there are posts about how to do a provisional (or invisible) cast-on popping up all over. They are all beautifully done, with lotsa pictures to explain and great results. With a crochet hook.
While I am all for using clever knitting techniques, in my opinion using a hook is unecessary hassle. And a lot of people have apparently trouble with the "easy" unraveling of the crochet cast-on; I am not ashamed to admit that I had a problem, too, when I tried it. I'd never go as far as unraveling a normal long-tail cast-on as sometimes proposed. Instead I do the lazy woman's (Alison is right. I had to correct it.) version with waste yarn and without the hook. Works just great.
Advance notice: anyone who visits this blog more often will know of my love-hate relationship with my camera. What can I say, the pictures are not as sharp as I would have wanted to. Sorry about that.
Lazy man's provisional cast-on
You'll need a piece of waste yarn, preferably but not necessarily the same gauge as the actual knitting yarn.
Make a slip knot using both yarns and slide it onto a needle.
Step 1: Hold the yarn as for the long-tail cast-on, with the waste yarn on top. Bring the needle to the front, bringing tip of needle underneath both strands. With lower strand make first loop on needle.
Step 2: Then bring needle from the back before upper (brown) and under lower strand (white)of yarn, pull up to make second loop.
Repeat Step 1 and 2 until you have required amount of stitches.
Turn needle and start knitting.
After knitting the last stitch, just slide slip knot off the needle and leave it. Knit on.
When all the knitting you want to do is done, it's time to get the live stitches off the waste yarn. There will be always one less than cast-on. Undo slip-knot. Insert needle into stitches one by one.
Pull out the waste-yarn. This is where we come to the one little draw-back of this method which can be easily remedied. If you look closely you will realize that every second stitch is mounted the wrong way. By knitting into the backloop of these stiches on the first row they will be adjusted.
And all this without unzipping anything but a slip-knot.