So you've bound off all of the stitches but the last one. You've got one stitch left on the right-hand needle, one left of the left-hand needle:
Slip the stitch on the LH needle to the RH needle purlwise (meaning the tip of the RH needle goes though the stitch from the top to the bottom, can also be thought of as from the back of the stitch to the front):
Both stitches are now on the RH needle.
With the LH needle, pick up the loop just below the stitch from back to front:
Now slip the last stitch on the RH needle back to the LH needle, purlwise again:
Two stitches on RH needle, one on LH needle.
Knit the two stitches together:
Two stitches left.
Then bind-off the second stitch as per usual. Break the yarn, allowing at least 6 inches for weaving in the end, and pull the yarn through the very last stitch:
Pull it all the way through and tighten it snugly.
The last stitch looks cleaner than if you had just pulled the yarn end through it without the other rigmarole, since it isn't just tightening in on itself anymore.
I love this little finishing tip to prevent an odd looking last stitch. It's just another one of those things to make your work look more professional (not that I don't already know you're a pro)!