I help people improve their credit scores so they can get a mortgage all the time so here's a quickie thumbnail of some of the major points. BTW having a positive balance in your checking account is good for you, but has zero influence on your credit score other than you'll have the capacity to pay your bills.
Creditors want to know if you also have the willingness to pay your bills.
First of all a credit score isn't a credit rating. Your credit rating is only about 1/3 of the score.
DON'T PAY OFF COLLECTIONS!! Paying it off starts the clock ticking again. It's as if the event happened when you paid the collection. The fact the collection happens is what drives your score down, not the fact there's a balance.
Have ACTIVITY UNITS. That means have payment activity on the credit bureau. Not all things report to the bureau so they don't count if they don't report.
The credit bureau doesn't know if you pay the minimum payment or pay it off, all they see is that you had activity. So charge your gas & meals and then pay it off or way down every month. Don't get in over your head. You can get a good credit score by having small accounts.
Since the credit score isn't a credit rating but rather a predictor of whether you'll use credit or not, it takes a certain number of open accounts to get & maintain a certain score. I've found it takes 3-4 pieces of open credit to get & maintain a 620 credit score. If you want a 700 score you'll need more pieces of credit.
No creditor reports to the credit bureau every month. They usually only report every 3-4 months although some only report every 6 months or once a year. This means the credit bureau is always behind and you need to plan ahead. It will take a while to make a difference in your credit scores.
Events such as paying off your car loan will temporarily raise your score, but long term the reduced credit footprint will lower your score.
Almost 35% of your score is your available credit. Keep your balances to 50% or less of your credit limits. But occasionally you will need to charge your card up in order to keep your credit limit. Under new credit card laws if you don't use it, you lose it. I recommend charging something you want to pay cash for and then paying it off a week or so later.
About 3 months before you want to buy a house call all your cards and tell them you will be buying a house and will need to buy a lot of stuff for it and you need to raise your credit limit. That's a free credit score boost.
You need to have a variety of credit types. The credit scoring engines break it out this way - mortgage, revolving, installment, & other. Just having a mortgage will raise your scores, but if you don't have a mortgage you can still get & keep a high score if you have both installment & revolving credit. Other credit doesn't help as much. I just had a customer that had 12 reported Tote the Note cars on his report and still couldn't get a high enough score to buy a house.
Oh, and a high score won't guarantee you a mortgage loan, there are other factors at play there.
Rebuilding credit
How to get & keep high scores