Formula for strong but lightweight paper-mache
I've always used slightly diluted white PVA glue (just enough water to make it runny enough to soak paper in). Recently tried using kitchen roll, which was durable and held its shape well. Tissue paper (as in artistic, not toilet roll!) is also nice - very lightweight, and can also go directly on top of chicken wire, if you preferred to make a framework with the wire. Careful if you're using dyed paper though; sometimes the colour leeches out of the paper, or just runs everywhere. 

EDIT 

Hopefully the photo shows it a bit more clearly. It's pretty tough, and holds paint really well. It smoothed fairly nicely, but we were in a bit of a hurry (and the bobbly texture actually worked quite well with fruit!)