Less is usually more.
1. Clear visual hierarchy
Headings, sub-headings, paragraphs…
2. White space
Give eyes a breather. A mistake that young graphic designers frequently make: they fill too much of the page with too much information.
Just like in public speaking, where you give the ears a break by having a short pause from time to time, in layout design you should give the eyes a rest by having white space.
3. Legibility
Use easily legible typefaces, have good contrast between text and background, lines not too long, texts not too small, enough inter-linage…
4. Alignment, consistency, simplicity
Think of a well-designed large supermarket, where the grid layout is clear, and the individual aisles are well and consistently signposted.
5. Golden Rule
Remove ink, wherever possible, within reason.
Whenever you add something, you are diminishing what is already there, and the reverse applies as well.
See this PDF page, where the diagram on the right has much less ink than the left — but is more visually attractive and more clearly conveys the information.
These “rules” can sometimes be broken, but you should do so knowingly.