Like others have previously commented, your rim is bent. You could try to bend your rim back and true it from there (hey, worst case scenario, the rim is still toast). The doorframe trick works if you de-lace the rim, but if you don't want to do that, try this: two pieces of 2x4" bolted down securely with enough space between them that you can slip the wheel in. That way you can bend the wheel with these without marring the braking surface too badly. If this sounds too confusing or daunting, get a new rim.