Instant in-store financing can demolish your credit score

"Would you like to save 15 percent on your purchase today?" We've all heard the pitch at the department store before.

This kind of instant in-store financing -- along with the offers of zero percent interest on larger purchases like appliances or furniture -- is a favorite among consumers.

But should you go for it? The reality is that opening in-store financing can actually demolish your credit score.

Here's why: A big part of your credit score is based on the amount of your available credit that you're using. Those store cards typically have smaller limits and it's very easy to max them out.

Ideally, you want to stay below 30 percent usage on credit cards of any type. For example, if you have a card with a $10,000 limit, try to use no more than $3,000 of it at any one time.