I was just wondering how some of you straighten your chassis's back out after bending and flexing them. I have been using a hammer to straighten out my chassis but that doesn't get it back to completely straight. Any tips would be great

depending on the severity, i either just use my knee to bend it back, or use a big vice.

Yea you cant rely on a hammer to get it back completely straight (well you can with the correct technique, but thats another story). The easiest way is to bend it back a little further than it needs to go so that it will return to its original position. Just bend it a little further each time until its there.
If its not creased, just bent then be careful in the vice, you DONT want to crease it. If it is creased then a vice will work great. Like dustin says a knee can work, as well as over the edge of a table for large bends. For smaller bends pliers (or a vice) may be needed. There are lots of diff ways it all depends on the type and location of bend.

a vice is my favourite weapon for chassis bends too

You can get a standard Dolly yes it is called a Dolly or a Hammer Dolly these are used in the Auto body trade to hammer out dents.Put the dolly on the flat side and use a flat face "bumping" hammer . You may have to shrink the metal after awhile efery time you bend it you will stretch it. You can find a cheap set at tool places like harbor freight.

"Dolly" -Vice! "Bumping hammer" -Standard claw hammer, will be fine for banging out a dent in an aluminium chassis. You will only stretch it if your "dolly" is right behind the hammer face. Heres a tip, hold the "dolly" on a spot next to the dent, and then hit it the dent down. This is typically called an "off dolly" hit and as long as you dont go crazy will not stretch the material. If you can get away with just bending it in the vice or similar I would do that first as you have more control...