Start by making sure that you're eligible for even a partial Roth IRA contribution. Do this by calculating your modified adjusted gross income (MAGI). To figure this out, you start with your adjusted gross income from your regular tax return on form 1040. Subtract from that any income that resulted from switching a regular IRA to a Roth IRA. Then add back the deductions and exclusions listed in IRS Publication 590 (available online at www.irs.gov).
If your MAGI falls in the phase-out range -- $150,000 to $160,000 for a married couple filing jointly, or $95,000 to $110,000 if filing singly -- you're eligible for only a partial contribution to the Roth IRA. A worksheet on page 28 of the 1040 instruction booklet will walk you through the calculation of contribution limits for MAGI levels in the phase-out range.
If you fall into this latter category, and you have accidentally contributed too much, my advice about recharacterization applies to you.