A recent book I read broke down the basics of design for those unfamiliar with design, like myself. These core rules can be distilled down to CRAP.
What is CRAP, you may ask? In this case, I mean the acronym, C.R.A.P.
C stands for Contrast. Don’t go halfway with contrast. Either make elements very similar or very different to make their relationships clear. Unrelated elements should have high contrast, and related elements should have low contrast. That contrast can take the form of sizes, colors, decorations, or whatever else. Just keep that contrast within reason.
R stands for Repetition. Establish elements as having the same relationship by using consistent styles between then. If you're showing a list of related links, keep them as similar as possible.
A stands for Alignment. Ensure elements are aligned with each other in a purposeful way to give the layout structure and focus. All paragraphs in an article should be aligned in the same direction along the same line. Also, avoid defaulting to center aligned text. If you don't have a good, specific reason for it, don't use it.
P stands for Proximity. Similar with contrast, don’t go halfway. Put elements close or far away to establish their relationship. Are users confusing one item with other, or think they're in the wrong groups? Put more distance between them.