Apart from the usual practice, practice, practice answers, maybe we can get some different kinds of answers from this question:
How do we ensure straight throws?
I’ve read around that for sleepers align with a tile on the floor, and for breakaways stand in front of the wall etc.
Are there any other ways?
I’m a very analytical person, so what I do is think very carefully about what I’m about to do and the physics involved. With that in mind, you can sort of “program” your throw. Very slowly run through your throw without actually throwing. Take everything into consideration, your feet placement, your shoulder, the angle your arm is at compared to your body, and that your fingers will roll out completely straight and at the right time with a good wrist snap to give the yoyo plenty of torque. There’s a lot of science involved if you think about it. In my experience, I’ve noticed that if any one of those things goes wrong, your throw is screwed.
Run your throw in slow motion over and over, this will help “program” your muscles into a straight throw and ensure that you don’t just revert right back to muscle memory when you go to throw hard right away.
This is just the way I go about it and it works for me. Hope this helps! (practice!)
Sorry, but that’s really going to just take lots of practice.