The IRS sets guidelines for how much you can contribute to an HSA each year. Following are the limits for 2011 and 2012:

You have until the tax-filing deadline (generally April 15) of the following year to make allowable contributions.

Excess Contributions
If you contribute more than the allowable amount to your HSA by accident or because you ended coverage in an eligible health plan, you will have to count the extra amount as taxable income and pay a 6% penalty.

To avoid paying taxes and incurring a penalty, you can fill out a Withdrawal/Distribution Request form (PDF) to have excess funds returned to you. 

Catch-up Contributions
You're eligible to make catch-up contributions if you are covered by an HSA-eligible health plan and are 55 or older at any time during the year.

Mid-Year Qualification
When you enroll in a high-deductible health plan and open an HSA before the first day of December, you can contribute the total allowable amount for that year. To take advantage of the tax savings, however, you must stay enrolled in an eligible high-deductible health plan for the following 12 months.

If you end your coverage under a high-deductible health plan, you should calculate the pro-rated allowable contribution based on the number of months you had qualifying coverage. If you're 55 or older, be sure to include any catch-up contributions you made to determine the pro-rated amount.

If your contributions exceed the allowable amount, you can fill out a Withdrawal/Distribution Request form (PDF) to have excess funds returned to you. This way, you won't incur a penalty or have to pay taxes on the extra contributions.