Some people starch their fabric, that can help many different situations. I've never gotten into the habit myself -- the secret(s) to my success is pressing and pinning. I pin about every 2", all the time. Yeah, it can be boring but it works for me. I press my seams open and also have to pin every leading edge so it doesn't flip over. Again, it's what works for me.
For longer seams it might be easier to layout it out on your ironing board (it's my largest work surface).
I'm always looking for "justifiers" places where it is clear what the measurement is and where the seams come together. When things are off-set a bit, I've got no problems making a little pencil tick another 4" down or where-ever the next seam is supposed to meet. You can do a pinch pleat or pin as well, when I'm matching a border to a top, I have the centers and quarters marked separately and then fit the two together according to the marks.
One final tip is to keep pieces as close to the same size as you can. If you have 6 blocks to sew together, you want to start with two together, so 1+1=2, then you can maybe go 2+1=3, or 2+2+2, but you don't want to end up at 5+1. We typically have the smaller piece on top but it doesn't have to be that way, for me with my open seams I put the side with the most seams up.