A cheap trick that I stumbled upon ties in with my crampbuster.
The crampbuster comes with a rubber band sleeve thingie that helps the crampbuster grip and stay more or less in place. It does slowly vibrate downward under pressure, so every few miles I do need to bump it back up into position. No big thing.
Anyways, I've found that if you overlap the rubber sleeve over the bar end just slightly, this will hold the throttle in place for a small amount of time, so if you need to flex your fingers or whatever it'll keep you at speed for a few seconds. It doesn't require much force at all to twist the throttle to overcome the overlap, as the band tension is very easy to break (the vibes through the bars will allow the throttle to come to rest given enough time, say a minute or so). This could be a temporary problem if the bike went down for some reason, but at that point you'll have much more important things to worry about besides the engine revving... (broken bones, not getting run over by the person behind you, etc.).
Also, several here are a big fan of the cat (caterpillar) O ring as a budget cruise control. Forum search is your friend.
