from what i understand you should have 3-4 credit cards that you charge things to (you can still pay them off) so they remain active....(unsecured aspect)
also, if it has been a while since you have had a secured loan (car or house etc...) you should try and get a secure loan, even if you just pay it off...this could be the biggest factor in why your score is low. You might try to get a HELOC....use it once a year to make a large-ish purchase, then pay it off.....that may be all you need to do for the secured aspect.
I think you are wise to want to raise your score. I'm in a similar position, but my score is now 750ish...I would like to try for 800+
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