Don't pay late, even by a day. Set up an automatic payment if you have to, to make sure you're on time.
Dormant accounts are the first to get cut or canceled, so if you want to keep your card, use it at least once a quarter.
If your credit card company raises your rate or lowers your limit, call and ask to be reinstated. Often a computer program makes these moves automatically, so often if you call and speak to a human being you can get the decision reversed.
Finally, you are allowed to place a letter in your file at the big three credit bureaus -- Equifax, Experian and Transunion -- explaining why your credit score has taken a hit. The letter is not normally used for this purpose, but you could explain that your circumstances have not changed, but the bank lowered your limit because of its troubles. Hopefully other businesses that consider you for credit will see that and understand.