The first thing that you need to consider is the time that is best to carry out the infiltration. You would probably have to decide whether you it is best to infiltrate during day time, or if night time will increase your chances of success. The place you are planning on infiltrating may have some event coming up, which can give you an excellent opportunity to blend into the guests and get in. There may be a certain time when the security of that place is relatively tax, thus creating a real chance for an infiltration.
It is extremely important to become aware of any defence that is in place to prevent a breach by unsolicited personals. Scan the location to see if the place is protected by barbed wire or a fence. Maybe the place has a backdoor that is used for taking out the trash or any other activity. Are there any dogs roaming the area to attack any infiltrator? Learning about the defence will allow you to look for a possible weakness in them, thus making it possible for you to enter the place covertly.
After investigating the defence and its weaknesses, turn your attention to the human elements of security and finding a weakness in them. You will have to spend a few days observing them to take note of their habits. Does a security guard smoke a cigarette while standing at a certain spot? Are there any alarms that the security personals ignore, since they are normally set off by animals? Is someone in a habit of taking a nap while on guard duty? Does any guard spend too much time in the toilet? Is there a guard with stamina issues? Observe these things and then work out a plan to exploit these habits.
If there are certain areas that you must cross to infiltrate a place, but doing so is bound to set off an alarm or get you noticed, use a distraction. This can be something as simple and subtle as getting a friend of yours to call a security guard on duty and keeping him busy on the phone, or something far more risky such as throwing something into the eyes of a pursuer.
Learn to blend into the environment and people. Do not attract attention and avoid making any sound. Stealth is your best friend in an infiltration mission.