Negative credit, other than bankruptcies, generally will remain on your credit report for 7 years.
If you have bad credit, start fresh and remain current on all obligations.
Although your credit situation will not be totally clean for 7 years, it can be in good enough shape in as little as 1 year.
If you own real property, pay a mortgage/auto loan/ student loan on a timely basis, this will help you rebuild credit quickly. Same thing with having a credit card. But be sure to pay the balances in full as opposed to keeping a balance. Only make small purchases and make sure you have money put aside to pay the entire bill off when you get it. Within 6 months you will probably get a credit line increase.
Your credit will be back on track in 1 -2 years, decent in 3-5, and excellent in 7. During the interim, you can get credit, just not at the prime rates. Although sub-prime lending has become a large area in its own right and the rates are not that far off from prime these days.