I've had some problems handing over specifications that were wireframe designs + explanation of elements, to UI designers/artists.
What tended to happen was that the first few screens created were indeed free interpretations of the loose wireframe spec, but as we got closer to the project mid-end, the designs tended to mirror the wireframe designs (which never were intended as actual layouts), leading to increasing amounts of iterations for each new screen.
This despite being very clear up front of the need of not only art but also design.
Recently, I was suggested that doing a wireframe in the first place might be the wrong approach. I was told I should create a hierarchy of elements for each screen and let the designer work from that.
My question is: Which type of specification is best in order to give the designer the best possible help creating a good UI with a great user experience?
Are there any sample documents out there that I could use as basis for my revised spec?
Clarification, this question is primarily about specifications for a contracted freelancer. Like DA01 says, it's best to work together from the start and keep iterating. However, in the case when you contract someone for the UI design, who might be on the other side of the world even, the rules tend to be quite different.