Storing horizontally is part of it. I remember years ago reading a thing by one of the manufacturers that the single best thing to do to keep the ink flowing is to use the pen on a regular basis. Doesn't have to be extensive use...maybe even just a few swipes to make a line or something. But the ink needs to be kept flowing or it will dry out in the ball point mechanism.
I know I've heard some pretty "out there" ways to get ink flowing over the years, lol! I remember one about taking the pen apart and blowing through the tube, or warming it under warm water. Sadly, my solution (due to lack of patience) is to just buy a new one. So now I keep my gel pens front and center on my desk and try to remember to scribble with them from time to time.