Before we get started, let's consider the ethical implications. Using a student discount when you're not a student is a little dishonest, so if you're a sparkling example of morality you might want to forget this method and keep paying full price. If you live life in the more common moral gray area, you might want to consider the following excuses. First of all, student discounts are most commonly awarded to college students who either are wealthy enough to afford college or are able to incur a massive amount of debt (scholarship folk—congrats, but you're a rare breed and therefore do not count). The idea here is that college students aren't necessarily stricken with poverty or will be more so after finishing their education. For that reason, the student discount might actually be better applied after a student has graduated. If you agree with this reasoning, welcome to the forever student discount club.
The Sticker Method
Some colleges update their student ID cards every year with a date as proof of current enrollment, but that's an added and unnecessary expense for most. Some just don't bother with dates at all, but a more common compromise is the sticker. Many colleges will provide a dated sticker each semester so students can prove they're up-to-date on their tuition and prove enrollment to companies offering discounts. Fortunately, these stickers are incredibly easy to forge after you've graduated.
If you just need a digital photo of your ID (for online shopping), Photoshopping a forgery is pretty simple. The example above is just a simple Photoshop forgery. If you need the actual ID card in all its physical glory (for in-store shopping), you can easily take your Photoshop forgery and recreate it with a regular inkjet printer and some sticker paper. You can get sticker paper in all colors and finishes at your local office store, and if you find it's too hard to recreate a sticker you can just remove the existing stickers to make your own version.