For the super trendy with super stylish mudguards:
Giles Berthoud Leather Mudflap
A sumptuous piece of cow hide that is 6mm thick and embossed with the famous Gilles Berthoud logo.
The flap fits to your mudguards by way of two 3mm allen key bolts - which are a work of art in themself (which have a hidden nylock dome head nut behind).
The distance between the two fitting bolts is 40mm centre to centre - so you will need wide enough mudguards to fit them. You will also need to drill two holes at the end of your mudguard.
A snip at £16 from here...
http://www.practicalcycles.com/userimages/procart78.htm
Or TBC may be able to sort you out with something more practical, or you could just homebrew ;-D