I work road construction in the long summer hours I have a lot of overtime in.
I think you're talking about the exemptions on your W-4. Your employer should be calculating your withholding based on the exemptions and your actual income. So...unless you have a history of owing money, another part-time job or a spouse that earns money too, you most likely don't need to do anything special.
Your income tax withholding is much higher when you work 80 hours instead of 40 hours. Even though you just doubled your hours, your taxes being withheld went up more than that. You may be tempted to adjust your withholdings so that you don’t have to wait until the end of the year to get the extra tax being withheld back. My suggestion would be not to try that. First, your employer may not let you keep changing the number of exemptions. Even if they allow this, if you make a mistake and you end up owing taxes, IRS can come back and file penalty and interest for insufficient withholding. They can also order your employer to deduct the maximum taxes from your paycheck every week going forward. It is not worth that risk.
If you are talking about W4 exemptions, you really want to plan on an annual basis, since your tax liability is based on your annual income. The IRS has a good withholding calculator on there website: http://apps.irs.gov/app/withholdingcalculator/ You'll need to fish out some of your financial data to use this calculator effectively.