Design frames functionality. Develop a lens of empathy (for your audience/end user) to look through while designing, and ask yourself questions they'd ask while viewing your design. When the good enough threshold is met (it works), additional modifications should add value to the target audience's experience (based on their pain/pleasure sensitivities (small text is painful (annoying) to read for some while a great color scheme is pleasant for others)).
I usually list all the attributes of the brand I'm working with, along with lists of the current graphic's/project's needs, goals, and objectives. I'm able to deduce what objective criteria is needed for "good enough" and accomplish it (a majority of the work) with the client involved in every step (to avoid common design-client subjectivity problems), they confirm what I'm making has what they need in it to accomplish their goals.
The result are several lists that show you how to look at the project/graphic through the eyes of your end users (lens of empathy). This is before any major graphic work, because I need to define exactly what the graphic work is first (so I don't waste my time, or the client's). The client ends up seeing through the same lens as I do, because we both discovered it together while figuring out the functions of the project/graphic.
That last artsy subjective 10-5% is framing the functionality of the graphic/project, adding details and tweaking everything (fonts, colors, sizes, positioning). I usually end up with grayscale website mockups that are 80-90% done, with the client appreciating information architecture and seeing the colors/font/style of a website as the top 10% of the information architecture pyramid we've constructed together. The result is a website/branding the client loves, because it's 'simple, obvious, and common sense' to them and it happens to look really nice too.
Heavy reading, but the book "Cognitive Psychology and its Implications" by John R. Anderson is a good overview of Cognitive Psychology, and provides a lot of insights as to how to design for our brains (good way to define your own objective design principles). Lots of great examples for UX tests too! Understanding why http://en.wikipedia.org/wiki/Checker_shadow_illusion happens, along with many other things, helps inform my decision making process when considering the needs and goals of a project.
When MP3 players were new Apple figured out how to frame the function of an MP3 player with good design, even though there were other MP3 players that functioned more or less the same.
Figure out the objective design criteria of your project/graphic, and have an informed sense of how our brains integrate sensory information so you can frame it in a way that your end user won't give a first or second thought to.