Any way to improve credit score w/out applying for a credit card??

I just very recently paid of all my credit card debt and I have been told to improve my credit I should get another credit card and pay on time (obviously) but I have been denied. Is there anyway I can improve my credit score without getting another credit card? and how long will it take for me to be approved for a credit card again? and how bad is my score if I wasnt approved for a credit card? :(