Basically, I just googled "flatten warped records" There are a variety of methods. It seems temperature has more to do with this than storing albums vertical or horizontal.
Some just place the record at the bottom of the stack and over time the weight of the other vinyl flatten it out. Most of the DIY methods involving applying careful heat and placing the warped album between two sheets of glass (1/4" or thicker), then letting it cool to a flat pancake. You do have to be careful, not to get things to hot as this can ruin the grooves. You can use the sun outside, however, an oven provides a more even heat.
I will practice with an album I don't care so much about.
One method..
http://www.wikihow.com/Fix-a-Warped-Vinyl-Record
