Try to understand why your credit score always fluctuates between poor and fair credit but doesn’t go up. For that you will need your previous credit reports and statements. Read them carefully and try to detect which financial decisions made it go up or down. For example, see if you changed the day you pay your bills or may be accrue some balance. Remember, the earlier you pay your bills, the better; and the more you pay, the better. You should make on time payments with all your creditors. You won’t see any credit improvement if you will be paying one credit on time and defaulting on the other. Also, calculate your debt to credit ratio. In a nutshell, calculate the amount of credit you are using. If it is higher than 30%, you need to pay off some balance.
As for high interest rates and limits, since you are writing that you keep your credit accounts in good standing, try to contact your credit card issuers and ask for a lower APR or request a credit limit increase. Most credit card issuer will agree to do so because they do not want to lose their customers.