The SSK is the most commonly used left leaning decrease but some people find that it never seems to lie as neat and flat as it's right leaning counterpart, the k2tog. One way to get a neater SSK (or slip, slip, knit) is to slip the first stitch knitwise and then slip the 2nd stitch purlwise, before knitting both of the slipped stitches together as usual.

And, hey presto - a nice, flat line of SSKs. 

Slip as if to knit then slip as if to purl