How can i improve my credit score?

a few years ago, like an idiot, i decided that i wanted to get a few credit cards. i got 3 store credit cards and a capitol one card. within a year i had them all totally maxed out, but i've worked really hard a paid them all off. i closed 2 of them and the others have a $0 balance. i also have a loan for my car and i was 2 months late on payments a few months ago. so now my credit is like 550. i don't have anything in collection, but i do have about 18k in student loans. what can i do to make my credit better??