I want to show you what I think is a great and simple provisional cast on.
You will need 2 circular needles, one can have a smaller size tip, or just a cable if you are using those handy interchangeables.
Leave a long tail, especially if casting on a large number of stitches. Attach your yarn to the lower needle or cable with a slip knot.
Holding both needles parallel, wrap the yarn from the back and over the top of the working needle. You will want it snug, but not tight - the image is shown loose for instruction.
Count the stitches across the top needle. End by wrapping around the bottom needle and bring the yarn to the back to begin to knit.
If you need a lot of stitches, the long tail will be available to add more if you have (somehow) miscounted, and you can unwind or add any extras.
I have shown the process in 2 colors for clarity, please notice that the furthest stitch to the right is on the bottom needle, and the stitch furthest to the left is on the top needle and that you remove the slip knot.
If you are working in the round, join your work at the end of the first row. I love knowing this new way to begin, and I hope that you will too.
I came up with this when casting on well over 250 stitches for this cowl, which will be published on Ravelry this weekend. Happy Knitting!
Edited to add: This is a great beginning for toe-up socks. I work magic loop style and have begun wrapping for the toe.
I hold, rather than attach the tail with a slip knot, wrap around the top needle and the bottom cable of the same needle.