There was a thread yesterday that would help you a bit. Click on this link. Scroll down to the picture Jan posted that show the stitches. That thread was about ribbing, where, as you look at the new row, what you see you knit. If its a knit, knit it--a purl, purl it.
For seed stitch knowing what the stitches look like will be a great help. As you look at the row you are ready to knit, work the opposite stitch. If what you see is a knit, purl it---if you see a purl as the next stitch, knit it.
You accomplish the same thing by having an uneven number of stitches and doing each row beginning K1, followed P1 alternating all the way across. But that is more the rote method. You will want to learn to not have to think " K, P, K, P, oh, my what was I on a knit or a purl?" If you learn to read the knitting it is always right in front of you telling you what to do and you don't have to remember. Much easier over the long haul. The picture will help along with what everyone has said.