1. When you pick up stitches for the sleeve, skip stitches evenly around. The reason this happens is that the length of a stitch is different than the width, so you have to compensate by picking up fewer stitches.
2. The very best, nearly invisible seam for stockinette is the mattress stitch. If your knit side is the right side, it's done with the right side facing. Amy had video on it in the finishing section. It's wonderful.
If the sleeve is in reverse stockinette--the purl side being the right side, you can still do mattress, picking up the lower part of the purl on one edge and the upper part on the other edge.
3. When I seam sleeves, I always start at the cuff so I know for sure that they're lined up. If something goes awry, any messiness will be in the armpit--not a biggy.
I go up on one edge and down through the other, and up through the first one again, leaving a tail, which is woven in later. Then I start my mattress and marvel at what a nice seam it makes.