If you know for a fact you have no vac leaks, the next step would be to check the idle air control valve. It should be mounted in the throttle body. It could be stuck open.
The computer controls idle speed by regulating air flow into the throttle body.
The throttle plate could also be stuck open. An eighth of an inch would do it. Make sure the throttle and cruise control cables are not pulling on the throttle plate.
Vac leaks could include the PCV valve and the EGR valve.