Since your income doesn't come in a constant fashion the employer should be compensating automatically. Usually the commission checks withhold at a higher tax rate so you usually have more than you need withheld naturally.
Try these tools:
http://www.irs.gov/Individuals/IRS-Withholding-Calculator
https://turbotax.intuit.com/tax-tools
http://turbotax.intuit.com/tax-tools/calculators/taxcaster
