This photo (not mine) shows the strap arrangement well. It's imperative that this strap is kept taut or the tank will be allowed to flex up and down and the front bracket will almost certainly fracture, sometimes resulting in fuel leaks.
As Planenut mentioned, some of us have reinforced the rather flimsy L brackets that are welded to the tank with secondary aluminium brackets bonded over the originals.
Front bracket:
Lower bracket:
The take home message is to regularly check that your straps are correctly tensioned!
