A key feature of any radar detector is how many bands it can detect. Even the best radar detector can't detect everything, but you want to make sure it works against the devices in use in your part of the country. There are three major bands in use in the United States today, X, K and Ka bands. In addition, many law enforcement agencies use lasers to measure your speed. For the highest amount of protection, find a police radar detector that can determine when any of these four types of speed measurement are being used against you.
Mounting your radar detector in the proper location can greatly affect its effectiveness. Clipping it to your visor is the simplest solution, but being too high up reduces the chances that it can detect radar and lasers used to measure your speed. Putting it on your dash or low on your windshield helps you detect more bands, but it can be a visual distraction. Both high and low placements have advantages and disadvantages, so pick the one that feels most comfortable for you.
While even the best radar detectors can't protect you against every sort of speed-measuring device, there are features that can help protect you when you drive in the city. Red light cameras are increasingly common, so if you're in an area that utilizes them, look for a radar detector that can warn you of their presence. In addition, try to find a device with separate urban and highway modes. This can save you from having a high number of distracting false positives.