I have a really simple plan for improving/maintaining my good credit rating.
I pay my bills on time.
Seriously, I am a simple gal - I figure, if you have to play games to get a good credit score, I don't want to play. How hard can it be? Buy stuff, sometimes on credit - I like the "x months same as cash" deals; this is a good way to build credit without paying interest.
We do that a lot but generally only one loan at a time. We're going to have two of those going soon, 'cause we NEED a new a/c installed! But both are due to be paid off soon, it's not going to be a big problem.
if I carry a credit card balance, it's for a really good reason - a short, short term loan vs. unplanned spending. Therefore I know it'll get paid off in 3-4 months. If I need a loan longer than that, I get a home equity loan.
Sometimes I mess up and pay a bill late, but generally all that gets me is a late fee. I think you have to be >30 days late before it affects your credit rating.