Delinquent Debt - There's not much that will look worse in your credit file than a delinquent debt that was handed over to a collection agency. That means the creditor has written off your account, and sold it at a deep discount to get the "bad loan" off their books. Not only will these events absolutely make their way into your credit reports, but just consider how any potential lenders in the future will feel when they see these.

Bankruptcy - this may be the absolute worst credit event for your credit report. One reason I say this is that this will be seen by everyone reviewing your credit file for at least seven years. It's already hard to get mortgages and loans now; you can pretty much give up on these with a bankruptcy in your file. Even loans that are still made available to you in the coming few years may be at the highest rates offered. That means your next car may cost a lot!

Missed payments - How can potential lenders in the future have any confidence approving your loan applications when they see a history of you NOT making payments? Missed payments for any significant loan will certainly get reported to the credit bureaus. This is actually a very common scenario but so senseless! Call creditors and work something out; offer a partial payment, offer some now and some on payday, work something out to make a timely payment. Missed payments will always hurt your credit rating, but can often be avoided with a minimum payment or a creditor willing to work with you.

Applications for credit - keep these to a minimum to avoid dragging down your credit. While nowhere near as bad as the other items mentioned here, applying for too many loans, credit cards, lines of credit will hurt. Usually just applying will result in a hard pull of your credit - no problem until you start to get several of these. If you've applied for some loan or credit card recently, try to avoid doing so again for six months while you work on gradually raising your credit scores.