I like to establish the general art direction early in a project. To avoid going overboard, I limit the initial collection to just one background, a few animated characters, and some weapons and objects. Just a few polished assets can make an impact on the programming and design side of things. It can be motivating to understand the potential behind what would be a messy collection of primitive shapes and unlabeled buttons.
Normally background art is my greatest weakness, so I took my time with it by creating a dozen individual pieces that all come together as a single composition.