If you think your withholding won't be enough to cover the tax on all your income then send in an extra payment now.
Here are the 1040ES instructions and blank forms (where to mail is on page 4)
http://www.irs.gov/pub/irs-pdf/f1040es.pdf
Or pay directly on the IRS website. Payment options for paying your Federal income tax…..
https://www.irs.gov/Payments
