Let's look at the headers for the first request to a resource http://your.host.com/some/path/browsercaching/foo.cfm. The client request has nothing unusual:

GET /spielwiese/wollny/browsercaching/foo.cfm HTTP/1.1
Host: bluebox.computec.de
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Cookie: some=value; someother=othervalue;
Pragma: no-cache
Cache-Control: no-cache

The reply however carries three headers that tell the client a) when that resource was last modified (Last-Modified) and how long it should possibly be cached (Cache-Control, Expires):

HTTP/1.x 200 OK
Date: Thu, 21 Jan 2010 13:31:21 GMT
Server: Apache/2.2.3 (Debian) PHP/5.2.0-9~computec+1 proxy_html/2.5 mod_ssl/2.2.3 OpenSSL/0.9.8c JRun/4.0
Last-Modified: Thu, 21 Jan 2010 13:36:13 GMT
Expires: Sat, 20 Feb 2010 13:31:21 GMT
Cache-Control: max-age=2592000
Vary: Accept-Encoding
Content-Encoding: gzip
Connection: close
Transfer-Encoding: chunked
Content-Type: text/html; charset=UTF-8

Now if the client supports caching, it will include an If-Modified-Since header in its next request to the same resource:

GET /spielwiese/wollny/browsercaching/foo.cfm HTTP/1.1
Host: bluebox.computec.de
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Cookie: some=value; someother=othervalue;
If-Modified-Since: Thu, 21 Jan 2010 13:36:13 GMT
Cache-Control: max-age=0

Now all we'd need to do is read this header, determine the current last modified date for the resource, compare those two and if there's nothing new, we simply reply with a 304 'Not Modified" status and skip the rest of the page processing, so there's no reply payload. Our reply header would look like this:

HTTP/1.x 304 Not Modified
Date: Thu, 21 Jan 2010 13:31:26 GMT
Server: Apache/2.2.3 (Debian) PHP/5.2.0-9~computec+1 proxy_html/2.5 mod_ssl/2.2.3 OpenSSL/0.9.8c JRun/4.0
Connection: close
Expires: Sat, 20 Feb 2010 13:31:26 GMT
Cache-Control: max-age=2592000
Vary: Accept-Encoding

You can use this customtag to send and check the appropriate headers and control the actual delivery of the reply (store as browsercaching.cfm):

<cfparam name="dtLastModified" type="date" default="#now()#">
<cfparam name="cacheTimeSeconds" type="integer" default="2592000">
 
<cfset variables.strPageModified = GetHttpTimeString(ATTRIBUTES.dtLastModified)>
<!--- now set default expires and cache control headers --->
<cfheader name="Expires" value="#GetHttpTimeString(DateAdd('s', ATTRIBUTES.cacheTimeSeconds, Now()))#">
<cfheader name="Cache-Control" value="max-age=#ATTRIBUTES.cacheTimeSeconds#">
<cftry>
<!--- if there is an If-Modified-Since header in the request and the time is the same as the last modified date, send a 304 and abort--->
<cfif StructKeyExists(GetHttpRequestData().headers, "If-Modified-Since")>
  <cfset variables.ifmodDate = ParseDateTime(GetHttpRequestData().headers["If-Modified-Since"])>
  <cfif ParseDateTime(GetHttpTimeString(ATTRIBUTES.dtLastModified)) eq variables.ifmodDate>
  <cfheader statuscode="304" statustext="Not Modified"> 
  <cfabort>
 </cfif>
</cfif>
<cfcatch type="any"></cfcatch>
</cftry>
<!--- send the Last-Modified header--->
<cfheader name="Last-Modified" value="#variables.strPageModified#">

In your code you'd simply have to somehow determine what's your last modified date (in this simple case I use the modified date of the CFML template itself) and pass that along with the cache time in seconds to the cf_browsercaching-Customtag like this (foo.cfm):

<cfset variables.structThisFileInfo = GetFileInfo(CGI.PATH_TRANSLATED)>
<cf_browsercaching dtLastModified="#variables.structThisFileInfo.lastmodified#" cacheTimeSeconds="2592000">
Here you'd actually start with computing and outputting your reply if the request gets this far.