Disputes over security deposits are the most common disagreements between tenants and landlords. Due to this, states have laws and regulations concerning what landlords can use the security deposit for, and the timeline for when a landlord has to return the security deposit.
As in any legal dispute, if you believe that your landlord is unlawfully retaining or misusing your security deposit, it is advisable to check the laws of your specific jurisdiction before taking action. The rules concerning security deposits can differ slightly from state to state; therefore you should read your state's laws and guidelines.

Stop Problems before They Start
The easiest way to get your security deposit back is to foster a good relationship with your landlord while you are living in the apartment. If you get along well with your landlord, misunderstandings don't have to escalate into major disagreements.
Even if you have a stellar relationship with your landlord, it is a good idea to take photos of the apartment before you move in; this will be of great help to you if you need to dispute a landlord's claim regarding the use of funds from the security deposit to repair any damage. Also, doing a walk through before taking possession of the rental home is a great ideal.
You can also ask the landlord to inspect the premises before you move out. After the inspection, request an itemized list of anything that the landlord believes you need to fix before you vacate the apartment; this will give you a chance to remedy problems, or even dispute the landlord's claims (if the damage was caused by a previous tenant, for example).

If Problems Arise
If your landlord refuses to give your security deposit back, fails to return the deposit within the time set by law, or if you dispute charges that your landlord deducted from the deposit, the first step to resolving the issue is to contact the landlord (or his/her agent).
First of all, clearly state the problem and request an immediate refund. Then, send a letter stating the issue, by certified mail, and make sure to keep a copy; you may even want to consider hand-delivering the letter and having the landlord sign and date your copy, as proof of having received it.
If discussing the issue with your landlord fails to solve the problem, it may be a good time to find a lawyer to assist you.