It's always best to persuade the landlord to voluntarily return your deposit if you can.  But if you can't, you want you letter to help, not hurt any case you may file. A couple things to bear in mind when sending a letter about your security deposit:

First, it's common for landlords to defend security deposit lawsuits by claiming that they never received a written statement of the tenant's forwarding address.  By including an address for the landlord to send the deposit in your demand letter, you block the landlord from claiming later that they didn't know where to send the deposit.  

Second, by sending a demand letter, you help establish the landlord's bad faith if the case goes to litigation.  Courts sometimes give mom-and-pop type landlords something of a pass in security deposit lawsuits on the assumption that part time landlord's don't know their obligations under the law.  By sending a demand letter which explains the landlord's obligations to them, you help show that the landlord wasn't mistaken about what they were supposed to do.

Third, there's no formal, legal requirement that demand letters be sent by certified mail instead of regular mail, or even email.  The important thing is proving that the landlord actually received the demand letter - something that's easier to do with certified mail since you'll have a return receipt.  

Finally, when you write the letter it's important to bear in mind that your letter is likely going to be read in court if it doesn't persuade the landlord to return your money.  You don't want your letter to backfire by making you look bad.  Don't write like an unreasonable, litigious jackass. Filling a letter with a multiplicity of legal citations, or big words, or threats to file suit normally does more to hurt you than it does to help you.  

If your letter doesn't work

Hopefully the letter does the trick and you get your money back that way.  But if it doesn't, your best option may be to file a lawsuit.  If you'd like to talk to an attorney about your options in that regard, click here to request a free case evaluation.