Steve M, for a round trip, normally all you have to do is choose a IAH connection rather than the "direct" flight (via IAH) among the choices presented.

If the website (COOL, Expedia, whatever), doesn't give you the connection option, then switch to multi-city search, and enter the following flights:

SAN to IAH
IAH to ORD
ORD to IAH
IAH to SAN

Keep the layover times in IAH over the legal minimum connection time to retain your Rule 240 rights, and keep the layover times less than four hours to get the through fare.

You also lose a segment with a direct flight.

The only downside is paying the IAH PFC, which is not levied if you take a direct flight. The $3.00 or so PFC is well worth the extra miles, unless the connection point is close to the Great Circle route, which it isn't for SAN-ORD.