Should I wash it in cold water or warm water? Or does it matter?

You been on a diet hardman? :p I'm pretty sure both works but I only know how to with warm water. This is how I know how to do it, incase you did'nt know; Wash the t-shirt in really hot water (using colour-safe detergent to protect the colours from fading), once finished put the shirt in a your drier on the highest heat available, DON'T let it get totally dry, remove the shirt when it's just about dry and nothing more (drying naturally will allow the fabric to return to its origional size). Iron the shirt with an iron full of hot water and iron the shirt on hot steam. The last step may not be necessary although I think you should do it anyway, just incase, i'm sure it help with shrinkage. If it's still not a suitable size just repeat the process. Read the labels to see if it's 100% cotton, if it's not, it won't work. Hope this helps.

- Diesel

This is a really interesting question actually. I have several promotional t shirts I have gotten at political rallies in the past few weeks. Haha the perks of being a college student in an important swing state are immeasurable if you're interested in politics haha. I got tees from coal companies, John McCain, the RNC, the Family Foundation so far.

Anyways point is most promo tees come in large or XL, and I had the idea to shrink my tees but I didn't know a method to do so. Thanks for the good ideas Diesel. I'll try that because they're definitely 100%