If I had to pick a "Most Useful" technique from my bag of crochet tricks, I would say that learning how to do foundation (aka "chainless") crochet stitches is it. They are just so useful in so many ways - anywhere you need a stretchy beginning edge to what you are working on, I would always use foundation crochet.
Realistically, you can make any of the basic stitches a foundation stitch, but in reality I tend to use Foundation UK Treble/ US Double the most frequently. Its a great start to ribbing on hats and mittens worked from the bottom up or any garment that uses alternating front and back raised/post stitches. It forms the start of the very popular Slouch and Bobble Hat featured recently in Mollie Makes, Prima Magazine and Irish Country Living from Hook, Stitch & Give.
For those of you not familiar with the stitch, I have a wee tutorial below. I am experimenting with the way I format tutorials here, to see if this is a bit more user friendly (feedback welcome). You can click through via the tutorial image to get to the PDF version.