[Superman]

When it comes to determining whether a specimen is too diluted, what they are looking for is a "human" proportion of creatinine in the urine. Creatinine is a sort of protein, or linked to protein. So yeah, don't drink so much that your urine looks like water. If this happens, then oftentimes it becomes time for an "observed collection," which is where the shy bladder thing comes into the picture. Further, to maximize the presence of creatinine, they say you should eat a nice steak the night before. This rumor is not confirmed, but not purely wives-tale either. Not a bad idea. Might I suggest a nice rib steak, grilled or BBQ'd with a subtle amount of appropriate spices.