How can i get approved for apartment rental?

if your credit is less than good and an apartment you are applying to get into denys you can writing a letter explaining your credit and how you're getting your credit score up help them to reconsider? if writing a letter can help your chances should you give it to them with the application (if you already know your chances for getting denied is high) or should you give it to them after you have been denied?........my credit is decent but my spouse has unfavorable credit.