There are many challenging interview questions, and "Why are you looking for a new job?" certainly falls into that category. Let's admit, it's a pretty frustrating question that can really throw you during an interview. But being prepared will make all the difference.
Whatever your reasons are, you want to be honest but discrete. You don't want to slam your current boss or colleagues, or tell your prospective employer that you don't make enough money. These are all huge turn offs and will get you quickly into the "no (and we never want to see you again)" pile. The most important thing is to keep it positive. Here is a list of perfectly valid reasons for looking for a new job that aren't going to get you anywhere in an interview:
"My boss and I don't get along."
"I don't get any credit for my work."
"I want to make more money."
"I haven't been promoted or been promised the things I expected when I started my job."
[See: 8 Tacky Job Search Faux Pas.]
Instead, the following approaches will help you advance further.