There s no definite rule. It s basically a historical problem. Words inherited from latin have this so-called mute 'h', while originally Germanic words, and most borrowings, have an "aspirated" initial 'h' (it s never aspirated in a phonetic sense, but I guess u know this...). Can have a look over there: http://www.aidenet.eu/grammaire01ac.htm

Don't worry! Even french people don't know when the "h" is aspirated or muet... A "H aspiré" in french doesn't sound like an english one...it's quite the same than a "H muet" We can chat together if you want to hear that...