You need to start with a strong creative brief that very simply answers who you are marketing to and what you know about them. Most advertising agency briefs have some sort of "single main thought" that you want to convey, along with up to three "proof points" that are the tangible benefits or features that support the main thought. Creating a good brief is an art form, and a good agency or a good strategic brand planner can develop one. Then you need to let a good team of creatives develop concepts.

The best ads tap into the emotions of their audiences. I've found that many times clients are concerned with saying everything that they feel they need to communicate, and the emotional impact gets lost in rational concepts that get lost to the end user in a 30 second spot or a display ad.

You can Google advertising creative brief to find examples, and if you like a certain brand or ad trying running a search for their creative brief. Ad Age has a pretty good article about briefs as well: http://adage.com/article/small-agency-diary/creative-briefs-simple/136711/.

Good luck!