Dig down to just above bedrock, and dig a long, level, 2x1 strip. You'll find diamonds within a few minutes. Each block destroyed this way exposes three potential diamond blocks. Finding diamonds is far more of a knowledge problem than a gambling one. Inefficient mining can make an 8 minute hunt for a diamond vein take over an hour. Also diamonds are a pretty hardy good, once you have a sword, pick, and armor, there isn't a huge reason to seek more until you consume those goods.