It sounds like the only derogatory mark on your reports is the late mortgage payment. I would suggest a good will campaign to see if you can get it removed. Even a single 30 day late can be 30-50 points off of your score.
Other than that I think as long as you keep your card utilization down under 8.9%, the only thing that will really reliably boost your scores is time.
Score factors that improve over time:
-Age of Oldest Account
-Average Age of Accounts
-Age of Youngest Account
-Number of Credit Inquiries
-Effect of late payments (will only lessen overtime, will continue to hurt your score until it drops off after 7 years, of if you can get it removed early).
Other things to consider:
-Some cards are use it or lose it, less likely with store cards, but it might be beneficial to use them for small purchases every few months.
-More cards are better to a certain extent. I would recommend looking into getting 1 premium card every year or two to upgrade your mix, and provide you better leverage. While inquiries and new accounts will lower your score, it will also make your profile more resiliant to changes over time.
-Credit is a tool like a shovel. Its meant to get used, and when used properly it will get a bit dirty. Don't worry too much about the ups and downs of your score over time. As long as you trend upwards you are moving in the right direction.
