Build credit after paying off debt

After some pretty bad financial decisions, my credit has taken a dive over the past years (student loans, hospital bills and a utility bill) However (!) I just payed off my last account in collections and i am now debt free (WAHOO!!) Just paying these debts raised my credit score 25 points, but I'm still sitting in the mid 500s. What can I do now, to build my score higher? I have no credit cards or other open accounts.