For this, I like to use the face up seam "Mattress Stich". It's easy, and I use it for all my sweaters actually.
Here is a link:
The second one down is the mattress stitch seam. I like to 'pull' the yarn every 5-8 stitches, and if you pull too tight and the seam buckles, just loosen it up before moving on to the next few stitches. What you are doing is picking up running stitches on alternating sides. It is such a nice seam technique that your stitches will disappear and it wouldn't even be visible if you use a different color to make the seam (although I don't suggest that!)