An extra leaf is always a good fix for a weak spring pack. When people have to re-arch springs, they must have been weak to start with - and you can be sure that over time, they'll go back. Adding a leaf can solve that situation.
I like air bags (the old Air Lifts were great) and as a second choice air shocks. Especially when the two sides are isolated and can run seperate pressure settings - makes for a quick preload change at the strip (also known as weight jacking).
Steve Metzger Tucson, Arizona