Most airlines' terms of transportation include a clause regarding purchase of "excess valuation" coverage for baggage; Here's USAirways':
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Excess Valuation: Baggage liability is limited to those amounts set forth in the above terms, unless an additional charge is paid. The excess valuation charges and maximum value allowed can be obtained from any US Airways ticket office. The additional protection (excess valuation) is not available for fragile or perishable articles which are not suitably packaged to withstand ordinary handling. </font>
Also note that if you're flying multiple carriers or codeshares the excess valuation purchased for one airline does not apply when the baggage is transported by another airline.