Say as much as you can, in the shortest possible way. Nobody wants to read the fine print, so keep the copy down and make every word count.
Outdoor works on scale and proximity, so your creative needs to work on both levels, from up close and from afar. This goes double for the copy.
Some colour combinations work together and some don’t. What looked good in the studio on a small screen might not translate as well when blown up 3000%.
To serif or sans serif? Both work of course, but whatever the font choice, never make your audience try to decipher what you are saying.
Outdoor advertising is geo-sensitive, so think about where your ad is running. How can you include the features of Outdoor location to elevate your creative?