The long-tail cast on is a perennial favorite of a lot of knitters, but it can cause some frustration, particularly when you have to cast on a large number of stitches, when it comes to estimating how long your long tail should be.
There are methods that can help you estimate -- such as wrapping the yarn around the needle as many times as you need stitches, or casting on 10 or 20 stitches, ripping them out and multiplying that length out over however many stitches you need -- but I have to admit that I usually just guess at an amount of yarn that looks like it would make as many stitches as I need. And almost without fail at some point I'll realize I don't have enough yarn for the stitches I need, but even then I probably won't do a proper estimate and it might take me three or four times before I get the proper number of stitches.
With that as your alternative, this method of casting on with two strands of yarn is a huge time-saver. It does give you two extra ends to weave in, but I think it's worth it to be able to cast on without worry or measuring.
To do it you'll need the knitting needle you're casting on to and two balls of the same yarn (or use a center-pull ball and work with both ends).
To begin, leave a short tail of each yarn and make a slip knot, holding both strands together. This will not count as a stitch and will be removed later.