Long time no post here - I'm giving out this tip once more, and thought to make sure everyone here has seen it too.
*** Are you painting your straight lines, freehand? ***
Please use this tip to not only have a better result but save yourself enormous amounts of time and sweating. The metal part of your paintbrush glides along the length of your ruler edge, safely raised above the page. In these photos I've drawn a line and modelling the brush, but you get the idea. Yes it does take a little bit of practicing, but believe me it is super quick and easy once you've got the hang of it!
Especially useful for perfect gilded straight lines and such-like.