I learned to use these from Rae Compton's "The Illustrated Dictionary of Knitting", a very useful book that I thoroughly recommend. The directions below are in my own words though. Rae says "As its name suggests this increase is almost invisible. It can be placed anywhere as long as there are at least two rows between it and another similar increase immediately above." --Kake
Knitwise leaning to left
Knit to the position you would like the increase to appear at. Use the tip of the right-hand needle to lift up the loop below the first stitch on the left-hand needle. Place this loop - without twisting it - onto the left-hand needle and knit into it. Knit the next stitch and you're done.
Purlwise leaning to right
Purl to the position you would like the increase to appear at. Use the tip of the left-hand needle to lift up the loop two rows below the last worked stitch on the right-hand needle. Purl into this loop.