The following code should work in most situations. This code is courtesy of Cameron Childress).

<!--- Anti-cache --->
<CFSET gmts = gettimezoneinfo()>
<CFSET gmt = gmts.utcHourOffset>
<CFIF gmt EQ 0>
  <CFSET gmt = "">
<CFELSEIF gmt GT 0>
  <CFSET gmt = "+" & gmt >
</CFIF>

<CFHEADER NAME="Expires" VALUE="Mon, 06 Jan 1990 00:00:01 GMT">
<CFHEADER NAME="Pragma" VALUE="no-cache">
<CFHEADER NAME="cache-control" VALUE="no-cache, must-revalidate">
<CFHEADER NAME="Last-Modified"
  VALUE="#dateformat(now(), 'ddd, dd mmm yyyy')# #timeformat(now(), 'HH:mm:ss')# GMT#gmt#">