Active Directory and security are two terms that must always go in tandem, otherwise, half the benefits of using Active Directory are lost which is to have a secure database of users (and sometimes confidential data) that cannot be exploited by outside sources. Active Directory default security is only so useful and you can and should take pre-emptive and pro-active measures to secure your Active Directory contents.
Create an Active Directory security model to follow to the letter and do not deviate from it. One of the common issues with corporations who use local domains and therefore Active Directory is that many times their security models are very lax. Not only that, one time too many after a certain point in time, lets say three or six months, this model stops being followed due to being felt as excess bureaucracy and thus Active Directory may become open to exploitation.
Make an Active Directory security assessment in order to constantly evaluate the situation of your Active Directory hosting machines. The best thing to do other than trying to install security and hope it stands up is to play the Devil's advocate and attempt to breach your own system, which is something several companies specialize in. Assess the level of security by having professionals analyzing the defined policies and by performing both automated and manual penetration tests to see how vulnerable is the network.
Establish proper Active Directory security policies since a improperly configured policy may be enough for a user to take advantage. All it takes to break a network is something improperly configured and this holds true as well for an Active Directory network security system. Be constantly sure to see that no user, or group, has any extended privileges other than necessary for productivity, since these are key holes through which hackers for example can exploit their way in.
Constantly examine the Active Directory security logs as it is imperative to understand what has been happening, when and how. Common error is to not monitor development of the Active Directory and all the actions that are taken with it, so it becomes very easy for penetration actions to take place amidst common user actions. Be sure to constantly verify on a daily basis for any anomalous entry attempt on the Active Directory logs.
Lastly, but not least, use the Active Directory integrated security. There is absolutely no reason to not rely to the integrated systems for security from alarms to automatic forms of self-protection, but this will only prevent so much damage so only use this as your foot hold. Establish better protection systems to keep your Active Directory system intact.