The standard Fairey overdrive is the simplest solution - you can fit one in a couple of hours without removing the gearbox or even the seatbox if you're feeling really idle - and you get an extra gearstick! Imagine!
However, beware. A good, quiet one is like gold dust. They double the number of ratios in your box, but they can leak and get very hot on long runs.
I wouldn't want to be without one, but I have been very seriously contemplating going down the Ashcroft route (see vag_landy's link above).
There is so much less to go wrong.
Hope this helps.
