How to keep your web designer happy?

you need to reward him by achievements. If he will make a good job, the site's income will rise, so maybe you should reward him by counting  entrances to the site. for example for every 10,000 entrances 100$ addition for his salary.   

Deliver all content in time and double check that you have the most final stage of information done before you pass it on to them. It's sometime really hard to change things after they were already put in a specific place \ design.

Have a clear idea of what you want to achieve from the site, fully understand the direction the designer has taken from you, have regular checks and deadlines to make sure that everything is still on course and as you want it to look. Communicate concisely and keep notes of all requirements.