Depends on what level of security the base has. If it's a top-secret officially-denied base conducting illicit operations, the security will be very heavy. The US government, for instance, hires sub-contractors as security for bases like Area 51. Usually it's a firm called "Wackenhut", and their security is top-of-the-range. The outskirts of the base will be patrolled by two HMMWVs (High Mobility Multi-purpose Wheeled Vehicles; otherwise known as "Humvees"). Both of them will have gunners operating the machine gun affixed to the back of the Humvee. It's the patrol's job to make sure no-one can approach the base without authorisation.
The base itself will have watchtowers. Usually one in each corner. The guards in these will be armed with fully-automatic machine guns, usually a FN P90 which has a blazing rate-of-fire of 900 rounds per minute. Their job will be to tackle anyone who breaches the perimeter patrols.
The base itself will be secured behind a fence, usually ten feet tall with a row of razor-wire along the top. In some extreme cases, the fence will be electrocuted. Warning signs will always exist in this case. Inside the base will be a phalanx of highly-trained ex-Special-Forces soldiers in high-grade body armour carrying more P90s. This is your main problem. These guys are the backup-backup, and they do not take kindly to intruders on their facility. Their positions and movements will be dictated by the base commander. He sits in his chair in the surveillance room and watches the various cameras around the base. The problem is that he knows exactly where you're going before you do. You might want to consider neutralising the cameras around the base before attempting to break-in.
This is how I'd do it: I'd have a team of men ready to storm the building. I'd place two snipers on the northern and southern sides, one thousand yards away from the building so the patrolling Humvees can't see them. First port of call would be to remove the patrolling Humvees. Simultaneous fire from both snipers to take out the drivers and their gunners. Since the base is over one thousand yards away, the sound of the rifle will never carry that far. Which means your men retain the element of surprise.
Next, the snipers will take out the watchtower guards. This is where the shit hits the fan. It won't be long until their bodies are discovered, and then you have a problem. This is where your guys start to charge towards the facility, only to be met by the phalanx of soldiers. While your ten guys engage their ten guys, the leader of your group slips quietly away and finds the surveillance room and the base commander. He takes the commander hostage and uses him to convince the other soldiers to put down their rifles. When they do, your guys blow them away.
Hey presto, you have access to a military base.
Hope this helps.