These guidelines are to help all of us make UI reviews as straight-forward as possible.
For a detailed process for UI changes (link to Curtis' suggestion... can't find it on the wiki yet.)
Starting your UI branch
What is this page trying to tell me
Although it may be out of the scope of the current branch that you are working on, it is always worthwhile getting the big-picture... and there might be small improvements outside the scope of your bug that will make a huge difference to the user experience. (Maris' to add notes)
Submitting your branch for review
Create a screenshot or screencast
The UI review process will be much faster if the reviewer can view your change without having to merge and run your branch. Sometimes this can be as simple as attaching screenshots to the bug (this also allows other interested people to comment).
If your UI change involves a behaviour, consider creating a brief screen-cast using gtkRecordMyDesktop and attaching it to the bug. Again, this will again help the reviewer and other interested people from commenting without having to merge your branch.