If you're running late with your taxes, you're not alone. But if you can't file your tax return by April 15, you shouldn't just sit around doing nothing.

Filing for an extension is a great first step, but it won't let you off the hook. You can be late filing your returns, but Uncle Sam will take offense if you're also late actually paying your taxes.

Extend away
To get a six-month extension of the due date for your tax return, you'll likely need to file IRS Form 4868, Application for Automatic Extension of Time To File U.S. Income Tax Return, by April 15. On the form, include the amount you expect you'll owe in taxes. (If you expect a refund, all the better; you won't need to enclose any payment.) You may also need to send in a few other forms.

Note that you'll owe interest on any underpayment amount, and you may incur a late-payment penalty fee on top of that. Fail to send in your return within the extension period, and you'll likely trigger a substantial late filing fee as well.

But don't put this off!
The IRS may give you a grace period if you're tardy with your 1040, but when it comes to your IRA contributions, its deadline doesn't budge. You have until April 15 to make IRA contributions for the tax year 2009. After that, you're out of luck -- and your retirement could suffer for it.

Contributing the $5,000 maximum to a traditional IRA (or $6,000 for qualifying taxpayers 50 and older) will reduce your taxable income and shrink your tax bill, while helping you to save for retirement. Making that contribution to a Roth IRA, meanwhile, will let you build a nest egg tax-free.

Pick the right stocks for your IRA, and you could enjoy amazing returns. Strong S&P 500 performers such as Staples (Nasdaq: SPLS) and Johnson Controls (NYSE: JCI) have turned their explosive growth in their respective industries into big profits for investors. Both companies have seen their stocks rise 18% or more annually over the past 20 years, turning an initial $5,000 investment into more than $135,000. In a Roth IRA, you wouldn't pay a penny in taxes on any of that $130,000 gain.

Can't decide which stocks you want to pursue for your IRA? Fortunately, there's no need to rush. As long as you deposit your contribution in cash by April 15, you'll have all the time you need to invest that cash in the stocks of your choice. (Just don't wait too long, or your stocks might grow without you.)

Whether your procrastination hurts you a lot or a little is up to you. Follow the IRS rules for extensions, and feed your IRA on time, and you'll likely do okay.

Visit our Tax Center to get lots of money-saving tips and practical guidance.

Dan Caplinger explains how you can still save thousands on your taxes.

Longtime Fool contributor Selena Maranjian does not own shares of any companies mentioned in this article. Staples is a Motley Fool Stock Advisor pick. Try any of our investing newsletter services free for 30 days. The Motley Fool is Fools writing for Fools.