Documentation and references. You write things down as much as possible, and then include some visual examples (which you may just google, but be damn fucking sure they don't end up in the final product). For example, for the above image of AoE-Online you could note that all buildings look like they have been squeezed in…
this. if you have a team working together, someone will have to take the lead in defining the style and ensuring it is maintained. you can write fancy documents all you want, people tend to not read the manuals. somebody has to keep the watchful eye. i find this very important when it comes to say differences in texturing…
This is good advice. Like everyone else has said: 1. Decide an art style, using CONCRETE EXAMPLES for reference 2. Collect that reference, and any other important decisions, into an art bible that everyone has access to 3. Make it someone's job to make sure all assets fit the art style 4. If the assets don't fit the art…