Plane Talk: Tips on Avoiding Baggage Problems | Department of Transportation: "Avoid putting the following in checked baggage:
Valuables (cash, jewelry). Don't rely on suitcase locks; they are easily defeated.
Critical items (medicine, keys, passport, tour vouchers, business papers).
Irreplaceable items (manuscript, heirlooms).
Fragile items (camera, eyeglasses, glass containers). If these must be checked, wrap them carefully in padding.