I agree with both Christian and JtotheA. When you get the tax rebate don't consider it a part of your normal income.
Like Christian says, put the money into savings. While I personally disagree with this tax rebate/advance, this check is coming in the mail. So I am personally sticking mine into my savings account so I can pay it back next year, plus whatever taxes come on top of it.
Another thing you can do is (if you owe this year) send the check back to the IRS; it will cover part of what you owe.