I spent far too much time thinking like a criminal today. This was inspired by seeing how little attention security paid to a police officer on a bicycle entering what should be one of the most secure areas in Washington.
Security guards/police officers travel on bikes near the target. First, attack one, take a complete inventory of their clothes, badges, and accouterments, then let them go. Or kill them, depending on the flavor of the story. Then repeat the process, only this time bring approximate replicas of the entire inventory. Steal the police officer’s clothes/ badges, etc. (let him keep the underwear, if you prefer) and burn the replicas. If we’re keeping the officers alive (preferable) make sure the officer sees the clothes being incinerated and believes that they are his. Then make it appear that the entire purpose of the exercise was to humiliate him- duct tape him to the back of a public transport or something.
Then, at the invader’s leisure, use the clothes/badge/etc. to bike ride straight into the high security area. All the obvious things should be done as well, of course- stake outs, steak outs (less known, but very important), learning the ins and outs of the place as well as the routine.