Raise your hand if this situation is familiar: you graduated from college in the past few years, and you are absolutely drowning in slightly over-sized, no longer wearable souvenir t-shirts from every high school and college event or activity you ever participated in. I've got sports t-shirts from high school, service project t-shirts from high school and college, club t-shirts, you name it. I even have a t-shirt that my Brownie troop made in elementary school. And I don't--or at least shouldn't--wear them anymore. I've already turned half of them into housecleaning rags,* but there are still a bunch left.
What's a crafty girl who hates throwing things away to do? Turn them into a rag rug, of course!
To that end, I've spent the past several evenings cutting a pile of t-shirts into long strips and rolling them into balls. Luckily nearly all the shirts are part of the same gray/white/blue/navy color family, so the finished product will look nicely unified.
To cut the t-shirts into strips, I started at the bottom hem and cut the body of the shirt into a long, continuous spiral all the way up to the armpits, rolling the strips into a ball as I went. I tried to keep the strips about the same 1" thickness, but didn't really sweat it. Perfectly even strips of material aren't necessary, but try to keep them all between 1/2" and 1" in width. Wider than that and it'll get harder to catch with your hook, but going much narrower might make your "yarn" too weak. Now that I've started crocheting, I would also recommend cutting off and discarding the bottom hem entirely, and starting your cut just above it. The double thickness of the fabric is really hard to stitch with and makes the final product lumpy.
A little bit of printing on the shirt is okay, but it does make the fabric a little stiffer, so avoid shirts with very large overall designs, or anything that feels stiff in your hands. Since the fabric tends to roll in on itself as you crochet, printed designs won't make the final product look weird.
For the first few t-shirts, I also cut the sleeves into strips as well, but now that I've started crocheting, I don't think it's worth the effort--it doesn't make a very long length of material, and all that joining really slows the speed of my crochet. But if you don't have that many shirts to start with, it might be worth it for you.
If you don't have t-shirts, old bed sheets, or pretty much any other scrap fabric, will also work. It is a rag rug, after all!
*Pro tip: old t-shirts make great cleaning rags. They're soft, absorbent, tough, and if they've been washed enough times, almost totally lint-free.