Cross-examine the officer about his radar training
To use radar, an officer must complete a 24 hour peace officer standards and training (P.O.S.T.) certified course. Ask the officer if they attended a P.O.S.T.-certified course, and, if so, how long it was. If they answer "yes" to both - which they will - ask them for documentation proving their certified status.
Radar unit must be independently analyzed
To be admissible at trial, the radar unit must have been examined and approved for accuracy by an independent testing lab within 3 years of the date you were issued the ticket. Again, ask the officer to produce written proof this was done.
Require proof of examination by an independent lab
The unit must have been examined by an independent testing lab within 3 years of the date of the stop for any alleged evidence ascertained by it to be admissible.
Again, demand documentation proving this was done.
How was the unit calibrated?
When police acquire a radar unit, tuning forks come with it. Each unit has a pair of tuning forks designed to test the unit's accuracy. Tuning forks are the recommended method for calibrating radar by both the unit's manufacturer and the national highway transportation safety administration. The user's manual also states tuning forks are the preferred method. Ask the officer if he calibrated the unit before stopping you, and, if so, what method did he use?
When was the unit last calibrated?
Just as there is a recommended method for calibrating radar, so there is a recommended time for calibrating radar. N.H.T.S.A. and the radar manufacturer both recommend calibrating the unit after every stop made by the officer where radar was involved. The Supreme Court's of both Minnesota and Wisconsin have gone so far as to rule that a conviction for speeding cannot stand if the only method used to calibrate the unit immediately prior to the stop in question was with the unit's own test switch instead of with tuning forks.
Request a certificate of accuracy for the forks
If the officer used tuning forks request a certificate of accuracy for them. Both NHTSA and the case of Connecticut vs Tomanelli ('65) hold that without the certificate tuning forks should not be considered accurate enough to calibrate a radar unit.
Yours wasn't the only vehicle's speed read by the radar unit...
A radar beam spreads out roughly one foot for every four feet away from its antennae (ie: patrol car) it travels. If the patrol car was only 400' away from your vehicle when radar alerted the radar beam was reading a distance of 100' wide - wide enough to read every lane of traffic - and the speeds of all the vehicles traveling on them - to your left and right, and in front of you as well as behind you. Maybe your vehicle wasn't the one making the radar unit alert.
Radar often reflect larger, further away objects...
A radar beam often generates a stronger reflected signal off of a larger object that's further away than from a smaller, but closer object. Therefore, the officer may have seen your vehicle when his unit alerted, but maybe his unit was detecting a SUV speeding along the freeway a few hundred feet behind you and not your little sportscar at all.