How to raise my fico credit score?

I signed up for a card a while ago and I have paid on it every time. It has been 3 months and my credit score is still the same despite paying my bill. I've had it for at least 6 months and my score has only gone up 20-30 points and for some reason it won't go up anymore. If someone can please suggest how I can get my credit score up without opening a bunch of credit cards I would appreciate it, thanks.

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