The simplest way to calculate is to take your total tax for 2011, divide by four and send in that amount each quarter. Note: This is not the amount due with your return or the amount paid in in 2011 if you received a refund but your actual tax liability.
IRS states that if in any given year you have paid in 'at least' as much as you owed the prior year there will be no penalties.
If you underpay your taxes during the year by I believe it is $500 or less, there is also no penalty.
And if you goof from time to time, with a reasonable explanation of circumstances IRS will not charge the penalties. (I.e. I earned more than I thought I had. or I thought my deductions were going to be higher.)
For more information see the irs.gov website and query for "Estimated Taxes"