According to IRS rules taxes are withheld based on your W4 filing. For bonuses there are two ways for your employer to withhold taxes on supplemental wages - see for reference IRS publication 15 - http://www.irs.gov/pub/irs-pdf/p15.pdf pages 15-17:
If your employer uses flat 25% rate for supplemental wages withholding - there is nothing you may do as your employer may not use different method of withholding at the same time.
You generally may ask your payroll person to change W4 - to reduce withholding. However the IRS expects taxes to be withheld as you are paid. If at the tax time you will owe less than $1000 and there will not be any problem to pay tax due - you are OK.
If you will owe more than $1000 - you may be penalized - please see details in the publication 505 - http://www.irs.gov/pub/irs-pdf/p505.pdf pages 48 - see also exemptions on the page 49 To estimate possible penalties - please fill the form 2210 - http://www.irs.gov/pub/irs-pdf/f2210.pdf