I came across this idea when we visited some neighbours who have also reformed their own cave by themselves. Its a bit of an obvious one but an effevtive workable idea.
When they laid there concrete floors they decided to insulate the floor too - to do this they used beer bottles. The bottles were laid top to tail creating a "floor of bottles" then the whole lot were covered in concrete. The end result was an insulated floor with the air trapped inside a few inches underneath. The difference, apparently is noticable.
Pro,s and cons: Obviously many many bottles of beer are required! this may take some time to collect them all but an enjoyable task in any case :)
This idea and others like it came from a book/website detailing ecological living. Simple ideas that cost very little and using recycled materials can be applied to caves very effectivly.