Color: As noted in the color choices box, some colors and schemes are not suitable for scientific posters. Stick to dark colors. Its fine to have more than one color in your font, but no more than 2-3 should be used.
Font Family: Comic Sans is never a choice. Ever. Also avoid curly, cursive, dainty fonts. Stick to plain fonts and serif fonts.
Size and Style: Each section should use the same size and style font. Your title will be large (85pt+) font and bold. Authors will be around 56+pt font. Subheadings are around 36+pt and can be bold. Body text should be 24+pt and captions 18+pt, with neither bolded.
Spacing: Leave some breathing room around your text. This gives a clear distinction between the sections of your poster. Don't feel like you have to fill every space with words or images. Less is more with posters.
Alignment: ALWAYS use left-alignment on your text. This prevents awkward spacing and stretching of your font.
When choosing colors for your poster, its best to use no more that 2-3 different colors. Try to avoid dark backgrounds with light text as they can be difficult to read. Light backgrounds with dark text tend to be easier to read and more visually appealing.
If you're using a KCU template, font colors and styles are already set for you. If you're creating your own poster from scratch, be weary of your color choices for fonts. Primary colors are the trickiest to use, but any bright color should be avoided.
Be weary of busy background choices as well. Its best to keep your background simple to keep the focus on your information. Using an image as a background can distract from the information itself and make it difficult to read.