Yarnover, Purl 2 together through the back loops - Yo, P2tog-tbl

Usually worked on the wrong side, this left leaning decrease (when viewed from the knit side) is cancelled out with a yarnover so the overall stitch count remains neutral.
The p2tog-tbl stitch can be quite tricky to get the hang of but it helps to keep the stitches fairly loose on the previous row.
1. Take the working yarn up and over the right hand needle, bringing it back to the front
2. Insert the right hand needle from left to right, into the backs of the 2nd and 1st stitches on the left hand needle
3. Wrap the yarn around the back of the right hand needle and bring back to the front, pushing the new stitch through the back of both stitches
4. Drop the 2 stitches off the left hand needle