There isn't really that much to it. The 音読み-reading is based on the Chinese reading of the same kanji, I believe. The 訓読み-reading is based on native Japanese words.
Onyomi-readings are usually combined with other onyomi readings, and kunyomi-readings are usually combined with other kunyomi-readings. There are way to many exceptions to use this as a rule, though.
Example: 場所、雨具、番組、残高、具合 etc.
These kinds of words are referred to as: 重箱読み（じゅうばこよみ）if I'm not mistaken.
Worth mentioning is the fact that some onyomi-words sound quite a bit more formal and wordy than their kunyomi counterparts.
Example: 買う/購入する、集まり/集合 etc.
Last edited Mon, 25 Jun 2012 15:30:49 +0000.