I have brewed about 90 of my own original recipes. In my experience, approach it much like cooking. Know your ingredients. You know what certain spices or herbs do to your soup or whetever you are cooking, so you need to know what different malts and hops do to you beer.
When using a product that I am unfamiliar with I read any description I can get my hands on and read other recipes that I know how the beer tastes and uses the same product. Feel free to expirement, but, in the interest of not ruining a batch of beer, go easy with unfamiliar products at first. Its better to make a note to add more next time than it is to realize you put too much in and dump it.
The other side is to know your style guidlines. If you are shooting for an American IPA, for example, use ingredients that would be found in an American IPA (american malts and hops varieties). Also be aware of gravity parameters. Not that all beers must be made to a style.
The best source for ideas are other recipes you can find in BeerSmith or other books. When I am making a new recipe I will page through my books and cruise homebrew recipe sites for inspiration and ideas.
As you use some ingredients you will learn which ones to use and which ones you like.
I love making and trying new recipes. It is almost my favorite part about brewing. It is a way to make the beer my own. Be creative.