When making large mocs:
DO make sure to stay with what looks good. If, when you starting to build away from the starting spot, it begins to look rough, cluttered, and junky. Stop building. Think smaller, or start over.
DO NOT make a large scale moc, and ignore any concept of color scheme, flow, balance or proportion. Many people believe that in a larger scale moc, the fact that it's big, means that it doesn't need a color scheme. These people ignore this basic guideline to make it bigger. Making it bigger, doesn't make it better.