Just to add.

Always know what the DOFD and SOL for each type of an account.

Never send a DV if it is within SOL unless you can pay the entire account in full.

Never close an account with a balance on it.

Never talk to a CA.  Always correspond in writing.

Paid and unpaid collections and Charge Offs are scored the same by FICO.  Charge offs with a balance that are paid will help your utilization.

Keep utilization below 10%.  For optimum score results keep it below 3%.

Do not give up.  It may take a while but your scores will incease.