You can request a free transcript of your return by mail using Form 4506T or 4506T-EZ. A transcript lists all transactions related to your return for that year, including payments and any adjustments.
The request can also be made by phone at 800-908-9946 or on the IRS website. Transcripts are usually available for up to 10 years of returns.
If you want an actual copy of your return, use Form 4506. There is a fee for each year requested.
Copies are usually available for the current year and the previous six filing years. But this is not guaranteed, so keeping a copy of all your own returns is the safest way to insure you have them if needed.