Hi Kim,
good question - the main thing that I do differently on the lower strings is that the lower I go, the further my hand moves towards the bridge - as I go across the strings, as the string gets higher, my hand moves towards the neck.
On the Lowest string, my thumb is normally planted against either of the pickups or the end of the neck, depending on the kind of sound I'm going for. Beyond that I don't do anything special...
cheers!
Steve
www.stevelawson.net