When you tie a bow, does it turn sideways and look like this? Yuck!
I have found that the 'natural' way I tie a knot and then a bow will always set the bow sideways. I have to look at my knot and make sure the top tail is in the right place so my bow sits straight! So simple, so effective.
Tip: If you are making a bow without a knot - like sewing ribbon onto an item for a stationary bow - sew the ribbon on vertically so you have a top and bottom tail, then tie the bow (no knot) as directed above and your decorative bow will also be horizontal.