Three ways come to mind. The first is to get it straight to start with.

That's not much help and is actually easier said than done. The Nikon D3 has a built in horizon so even the pros need help so you shouldn't feel too bad about it.

Method two is simply to rotate the picture then crop out the useful rectangle. The third is to use the perspective control. This is a very useful control that can be used to correct verticals on buildings. You'll need to play about with it as you will need to keep the sides and top square and straight and just adjust the bottom line.

Any other ideas folks?