Sorry, this page is still Very Much IN PROGRESS! So far, the main idea is to use plain white, or maybe cream, ivory, or other "very close to white" colors. Do not use other colors in order to stand out -- they will work... negatively! Use standard-sized paper and envelopes. The address and return-address should be neatly typed (i.e., applied by printer if possible); a label may be OK for the return address. Do not use odd textures; you may use slightly textured paper, like fancy stationery used to be, but do not use the ragged-edged, lumpy, bumpy, handmade (or fake-handmade) paper! Remember, it's supposed to look neat and professional.
The exception is if you're in some kind of artsy-fartsy field, in which case damn near all the rules go right out the window. My advice is aimed primarily at technical professionals, and other "reasonably close to normal" people.