A lot of people tie a stopper knot with the extra rope after tying a figure 8. I prefer doing the below method. Both methods are useful for making sure the short end of rope doesn’t slip through the knot. The trick shown below is also nice because it gets that extra end out of the way, making it easier to grab the rope and clip it in.
It’s pretty simple. After tying the figure 8, you take the free end of rope and stick it in between the loops of the knot (where noted in the picture).
UPDATE: After writing this, I found out that this is also called a figure 8 with an extra pass. Here’s a great page with step-by-step photos of how to tie this knot.

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I have heard it called the yosemite finish
Thanks for the great tip, simple and effective. Another thing I’ve noticed after I started using your method: if you don’t pull the loose end all the way through the knot but leave a bit of slack (like a small loop around the 8), it makes untying the 8 easier (just pull on the small loop to get the loose end out of the knot, and then the whole knot becomes less tight).