How to dry out the inside of a bicycle frame?
The most important thing to do to keep the inside dry and prevent excess rust is a proper drain hole on  the underside of the bottom bracket. Outside of issues with heavily salted roads, drain holes are enough to prevent meaningful rust. The next step is something like Weigle framesaver, or just a blast of aerosol lube every couple years in each tube. Framesaver is great against salted roads, but otherwise is usually overkill.