First of all, I join from the front, and don't pull too tightly. The trick to seaming an afghan is to keep the seams as flexible as the rest of the squares.
With a tapestry needle I pick up a purl bump at the very edge of the square (where the little nubs are created from turning your work). Alternate between each side gently weaving the edges together. Remember, DON'T pull the yarn too tightly. If you do, gently stretch the seam vertically to loosen it up.
Seaming takes practice, but I honestly think it is as important as the knitting itself. Give it time, and be patient. You can do it!