Do those clamps work in blind holes? I.e where there is no access to the back of the panel?
These little clamps are brilliant. Easy to make your own.
http://www.frost.co.uk/intergrip-welding-clamps.html
Also magnets across the seam whilst tacking help too.
I did spot this thing a few days ago and I'm not sure if it's a bit of a gimmick or not. Although the idea seems like a good one.
http://www.eastwood.com/perfect-panel-prep-tool.html
Joggle tool, you can get cheaper hand operated versions. There is also a hole punch on the head to creat puddle/spot welds on the joint. Works well enough but has the disadvantage of creating a panel gap behind for moisture to enter and corrosion to develop. There have been one or two debates on the subject versus butt welding.
Slightly different to a joggler as it does create the step. It seems to create a valley for the weld to sit in. bit like a v shape