When I went to my bank to get a small loan to buy my first bike I was told that I have a credit rating of somewhere between 800 and 850 (Credit is rated out of 900 up here). And that some people had been in earlier who had a credit rating of 500 and were approved to assume a house. With my rating I was told that I could outright BUY a house, and the bank would cover it. I was 19 at the time.
All I've done is use my credit card and pay it off at the end of every month. I keep a fair amount of $$ in the bank, and after I got my loan I paid it off. Getting a good credit rating is easy, fixing one... that can be tough.
Of course I'm generalizing from a single example here, but everyone does that. At least I do.