Courtesy of The Death Star Skunkworks!
While biking recently, my cycle computer pooped out on me. Today, as I investigated it's apparent damage, I noticed that the 17 year old plastic mounting bracket bolt had literally worn out from hundreds of kilometers of riding, on 4 different bikes. The sensor measures the amount of times a spoke-mounted magnet passes it; with that and the wheel diameter entered [26.5] it calculates your speed and mileage.
Thinking it busted, I waved the sensor across the spoke mounted magnet, and was very pleased to see the cycle computer was still functional.
After messing with the original mount and finding it next to useless, the inspiration came to me from ineluctables Instructable on making a wallet out of old mountain bike tires.
An old bike tire is a very useful item.

Step 1: Items Required

Here are the items you will need to get the sensor mounted on the front forks of your bike.
Pliers
Wire cutters
Hole punch or pokey tool of some kind.
Small and medium zip ties
2" slice of old MTB tire
2 or 3 slices of old inner tube.
Electrical tape
Black UV resistan Zip ties, small and medium. [Don't use the white ones, they go brittle quickly when exposed to sunlight /UV rays.]

Step 2: Test and Adjust

After you have gathered your supplies, take note of how close the sensor must be to the spoke-mounted magnet; if it is too far away, the sensor will not pick up a reading.
I used a small zip tie to attach the sensor to the middle of the 2" slice of tire. I then temporarily tied it to the fork using a rubber band to get the initial fit. The sensor remained too far away from the magnet, so... another of 1000 uses for old inner tubes surfaced!
I cut inner tube slices, the same size as the tire slice, and kept adding them under the slice of tire, until I got the wiggle room I needed.
I estimate that the sensor clears the magnet by <2mm or 1/16th of inch. I'm very happy that it works.

Step 3: Affix the Sensor to the Fork.

I then took my sensor assembly, and inner tube slices, and wrapped it around the front right fork of my bike. I used a medium zip tie to fix it loosely in place; this gave me the option of getting it placed just right before the final pull, and securing the whole assembly.
Now, as they say when a job is complete, Bob's yer uncle.
I imagine that the number of people using bike computers with a broken mount who read this is very small; no matter, it's good to share this Top Secret Stuff.