Can I request a lower interest rate?

I was able to request a lower interest rate on my Mastercard. They lowered it for a 6 month period from 18% to 0%. I told them that I was wanting to transfer the balance to a new card I had received that had a 0% for 6 month introductory rate and that is when they made me that offer. I would assume that is common practice to keep your business. During that 6 months, I was able to pay a substantial portion of the balance off.

How were able to approach them? Did you mail them a letter? Was the letter, if that was the case, sent via the mail or through email? Do you have a template of the letter?

Nope. No letter, I just called them and told them that I wanted to transfer my balance to a new card that I was approved for (actually, I was only pre-approved), and the representative offered to match the offer for 6 months if I thought that would help me reduce my balance and I took her up on it. It was a Sears Mastercard and the interest rate reduction was a huge help.