Gotta agree with the statements above. As an example, I'll basically start by cementing one end of the wing or fuselage. Once that has set, I'll start slowly and carefully working my way around the two halves while checking the alignment the whole time.
One thing I'll do is take the back edge of my Xacto blade and press it right up to the seam. That way I'll ensure that one half isn't higher than the other half and I'll avoid ending up with a "step" between the two halves. I hope that made sense.
Eric