I used to have trouble with that too, until I tried a different technique.

Instead of putting the right sides together and joining them that way, I now line the pieces up side by side, matching as closely as possible from beginning row to end row, and all other rows in between. It helps if you're able to block your pieces before seaming them.

Anyway, plastic seaming pins are good for temporary lining up of the pieces.

Then use a whip stitch (through the sides of individual stitches), not too loose, not too tight. If it's done right, you can barely tell the right side from the wrong side.