I guess that for my riding, I try to use as lower power setting as I can comfortably get away with, and when out with pedal only riders, never really ever get out of eco. This has been proven to really extend the range of the bike. All of that tends to go out the window when I am on my own though, as the climbing rate etc increases in speed, and I switch to tour. Unless my knee is playing up, I tend not to bother with the higher power settings.
Staying out of mud is also a good way of increasing efficiency. Not usually an option for me.
Choosing tyre pressures to suit terrain also helps. Like you I run very low tyre pressures when riding locally, but raise them for trips to the South Downs.
Changing tyres to suit terrain would also be a very big saving. Taking my Macina Lycan as an example, I run Hutchinson DZO tyres, and I can hardly pedal the thing above the legal cut off point, there is just so much drag from them. Switching to say the OE Rocket Rons, transforms the bike into a comfortable and easy (ish) bike to pedal above the cut off point. Actually tyre choice has to be my biggest efficiency drop.
Locking suspension on climbs I guess also helps, but I've never yet done that.
Ensuring that you are in the correct gear before starting a climb. The same when coming to a stop, and then pulling away again.
Keeping a bike well maintained must help considerably as well. That includes not leaving caked on mud on the bike, ride after ride, and maintaining a close check on the final drive chain.
I think that things such as the braking and skidding that you mention, are as much to do with good riding practice than anything else. Smooth riding in any discipline is going to make for a more efficient time.
