"With an extension, you can get more time for filing, but you will owe interest on any underpayment, starting on the original date. If you underpay by more than 10 percent, you may be subject to a penalty," says Mark Luscombe, attorney and CCH principal federal tax analyst.

If you find your expected tax bill is much more than you're able to pay, you should try to pay at least something. This will help keep down those accruing penalty and interest charges.

You also could go ahead and make payment arrangements if you know you won't be able to come up with your full bill in a lump sum payment. When you file for your extension, also file Form 9465 seeking an installment payment arrangement. You'll automatically get up to three years to pay the tax balance in monthly installments if the bill is $10,000 or less and you're current with previous-year taxes.

Electronic requests
Don't have a stamp for the form? Realized you needed more time after the post office closed? No problem. File Form 4868 electronically, either yourself from your own computer or, if you use a paid preparer, have that person file your request.

If you are making the electronic extensions request yourself, your tax software should include the form and instructions, but you might want to go ahead and complete the paper form as a work sheet and then transfer the information to your (or your preparer's) computer. You also will need your previous year's tax return, as information from that filing will be used to verify your identity.

You can pay any due tax with your electronic extension request by direct debit from a bank account. In this case, have your financial institution information (bank routing number and your personal account number) handy, too.

Or, if you prefer, the IRS will let you mail in any tax due (check or money order) after you've electronically submitted Form 4868. You'll find the mailing address for your state on Page 4 of the form's instructions. Use the one in the middle column, but in this case ignore the instruction to also send the paper form, since you filed it electronically.

Plastic payments also accepted
You also can get an extension to file and pay any tax due by charging it to your Visa, American Express, MasterCard or Discover credit card.

The IRS has contracted with two private companies, Official Payments Corp. and Link2Gov Corp., to handle taxpayer extension requests. Call them toll-free or go to their Web sites to fill out an electronic extension request and enter your charge card info:

• Official Payments -- (800) 2PAYTAX (272-9829)
• Link2Gov -- (888) PAY1040 (729-1040)

This method, however, will cost you more than just the tax you owe. Each company charges a service fee of 2.49 percent of your charge amount.

Calculate your tax bill carefully
Whether filing an extension request by phone, electronically or on paper, estimate your expected final tax liability as accurately as possible.

You can't simply decide to pay the IRS $100 knowing your final bill will really be closer to $1,000. If the agency later finds your estimate to be far off the mark, it could void your extension.

While tax law doesn't strictly require you to pay your tax bill in order to get more time to file, you should or you could end up owing more in the long run. The IRS will add interest to any tax bill not paid by the April deadline, plus a late-payment penalty.

And if you're due a refund -- and yes, even people who are getting back money from the IRS put off filing their returns -- an extension request isn't necessary. There's no penalty if you don't owe. But remember that you won't get your tax cash until after you actually file your return.

Find more tax-filing information and tips in Bankrate's Tax Guide.

Freelance writer Kay Bell writes Bankrate's tax stories from her Austin, Texas, home. She also writes two tax blogs, Eye on the IRS for Bankrate and Don't Mess With Taxes, and is the author of the book "The Truth About Paying Fewer Taxes."