Have you passed the Social Security wage base of $106,800 for 2009? Once you do, you save 6.2% in Social Security tax on every dollar of wages.
Instead of pocketing the cash, you might allocate at least part of it to a 401(k) or other retirement plan. Alternatively, you might siphon off more dollars to income tax withholding. This reduces the income tax you owe for ‘09 without any pain. It could even create a refund!