How to choose a PPC company (Pay Per Click Company)

Pay Per Click companies (PPC) seem to be ten a penny, but how can you choose which is the right company for you?

The first thing to determine is who are they actually working for? This may sound a strange question f you are the one engaging them, but many companies take a percentage of the PPC spend. This means the more you spend, the more they make. What incentive do they then have for reducing your spend?