Practice...and More Practice, but probally not practicing what you think... In order to get a nice straight verticle line wherever you want it you have to start with... a nice straight horizontal line, Make sure your wings are level. If your wings are not level when you pull into your verticle line or into a loop (or any manuver at all) then you will not have a nice line. The hardest part of flying aerobatics is the setup before the manuver, practice flying wings level and that will help your verticles