My method bockintheUSA is simple and cheap and takes about an hour once you get used to it
Go to the local bicycle shop and ask for their old inner tubes (most bike shops don't repair punctures just replace the tube)
Mountain bike ones are best as they are bigger
As they throw them away they will normally give them to you for nothing (if not try another shop)
Cut the inner tube either side of the valve so you end up with a long tube then split the tube lengthwise so you have a 2"+ wide strip of rubber about 1.5mts long
You then wrap this round the spring (spiraly) so it overlaps giving 2 layers all the way down
Cover this in any type of tape (I use either vinal or packing) 1 layer overlaped
This makes a virtually indestructable cover as the rubber just moves as you hit it and cushions the blow
Total cost is about 50pence for a roll of packing tape or £4 for the vinal but as a roll does about 20 springs its cheap
I must do a how to with pics next time I cover a set
Jason