Are they all going crooked or just one or two? Sometimes you get a shaft that is prone to bending. Sorta like a Gumby doll. You bend it one way and it stays, you bend it back and it stays. The only way to keep it straight is to neatly stack it on the fire wood pile and don't touch it with your hands, just a match!
Everyone's above advice is pretty good. I straighten with the Ace Roller, leave them over night, straighten again, leave them over night, then straighten, sand, stain and seal them. They stay reasonably straight if stored correctly and not torqued when pulling out of the target.