Keep purpose in mind (your purpose and conference purpose)
Consider audience (technical background, etc.)
Think "press conference" —max 2 minutes/2 pages of info
Some content can be omitted to be included later in your published paper (e.g. extensive lit reviews or bibliography, complex tables)
Emphasize most important results, rather than showing all results

Sell your content
Keep material simple
Be selective in what you present
You want passers-by (who are also eating breakfast, drinking coffee, and talking) to get something from your poster in 30 seconds
Use logical order (people are used to reading in columns (top to bottom) from left to right, or in rows (left to right) from top to bottom
Label sections to help guide, e.g. use research journal manuscript sections or some derivation thereof as appropriate to your content
In addition to title, authors, affiliation, and usually acknowledgements, you should include "what, why, how, results, so what?"

High contrast (e.g. dark text and light background)
Beware of large blocks of bright colors
Beware of dark or patterned backgrounds
Gradient backgrounds sometimes don't print well
Use color to emphasize/differentiate/add interest (not just because they're there)

Large enough to see from at least 5-8 feet, e.g. 14 (if printed at 200%) for text, larger for section headings and even larger for title (to be seen from 15-20 feet)
Simple font (e.g. Arial)
Italic and bold work better for emphasis than underlining
Keep to one (or very few) font types
Minimize use of all caps

Poster Production