Use more than one lock if you are worried about it. No one ever had their bike stolen by using too many locks. Also, maybe switch out any quick release wheels for bolt-ons.I got these for my commuter: http://goo.gl/ZAcPiF - they need a special allen wrench to come off (make sure you keep it with you though in case you have to change a flat!), which will deter all but the most aggressive bike thieves.
Also, make sure you lock it to the rack. I *almost* walked away from just locking my bike to itself yesterday...oops!