Have you filed your 2009 tax return? If not, there is a line on page 2 of the 1040 to include a place to collect your stimulus payment if you didn't receive it. If you have already filed, consider amending to collect this.
If you didn't receive an economic stimulus payment yet, you may be able to file for a payment still. Qualifications and requirements are given on the IRS website.
The Recovery Rebate Credit, which appears on the 2009 IRS 1040, 1040A, and 1040EZ forms, will determine if you are qualified to receive a stimulus payment for 2008.