Can't straighten rocket components

This is a little annoying quirk that I've struggled with whenever I'm building a rocket:

Half the time pieces refuse to go on straight.

Once the rocket gets into this state I usually have to scrap the whole thing and start over, because every part starts going on crooked (radial decouplers, fins, etc)

Why does this happen and how do I fix it?

Solution 1:

To make the parts go on straight in places like the one pictured, turn on angle snap.

Press C to toggle it, or, alternatively, there's a button next to symmetry at the lower left of the screen. It has a dot with a circle surrounding it when angle snap is off, and a dot with a hexagon surrounding it when angle snap is on.