Dear (name of customer service representative/ Branch Manager):
I have been a (Card Holder/store credit customer) for (number) years now, and I believe my credit standing with you is in fine order. My overall credit rating is also excellent.
My current credit limit is (amount), as it has been for (length of time). However, in light of the above, I am requesting that you raise my limit to (amount).
Along with my good credit history, my current financial profile is also healthy, so I anticipate no problems assuming the added responsibility of a higher limit.
My primary interest in raising the limit is to improve my credit rating.
Thank you in advance for your favorable consideration of this matter.
Your other complete details for reference.