You remind us of a good point, Daf--good preparation also helps reduce seamage.  Along with dry-fit, dry-fit, dry-fit!, cleaning the meeting edges, including sanding them as you describe, is a good idea.  Also, that makes me think off another method of avoiding seams, along the lines of Grendel's point about using styrene.  That's to use liquid cement and gently squeeze the parts along the seam, say, on a fuselage, and extrude a bead of melted styrene.  Hold the parts till that sets, and then you just scrape off the hardened bead.  Apologies if that's already been mentioned in the discussion.