I have one of these cheap and cheerful affairs: http://www.arkon.com/images/bike-mount/GPS032.jpg (Available for hundreds of eBay sellers for a couple of quid) I was forced to rely on using a GPS speedo app on my phone for a considerable length of time because the speedo on my BMW didn't want to work. This thing always stayed in place, never shook loose, never let rain in. And I did a lot of rainy miles, and rode on a lot of crappy bumpy roads. The bracket is easy enough to adjust, and even managed to get it clamped onto something as narrow as the cross-brace of my bars. Comes with lots of foam inserts to hold the phone tightly in place. Oh, and it has a little water resistant opening in the bottom (I believe the technical name is "rubber bumhole thing") for a power cable to go in.