Have you gone too long in between cleanings of your yoga mat? Did you know that you were even supposed to clean the thing in the fist place?
It is good to be blunt about these things: yoga mats are pretty gross. We cover them in sweat, rub our hands and feet all over them, and then leave them rolled up to let all of that introduced bacteria sit and multiply. Not a pretty picture. This is why regular cleaning of your yoga mat is essential to making it last and making your time with it a pleasant experience.
It's easier to blow out a match than it is to fight a forest fire; and it's easier to clean your yoga mat once a week than it is to salvage an old, stinky mat.
Prevention
Making a cleaning solution at home from your pantry couldn't be easier. First, choose either white vinegar or witch hazel as a solution and then make a 1:4 ratio with that and water. Some people don't love the smell of vinegar, and witch hazel is a good substitute. Put that in a spray bottle and spray it all over your yoga mat after your practice, saturating it enough to use a microfiber towel to wipe the mat down . Hang it to dry (without any sunlight touching it).
It's better to hang up your yoga mat whenever you can, rather than storing it rolled up as most people do. If you can, try to leave it unfolded and hanging any time other than while traveling. This will make getting into the habit of spraying your mat down with your homemade solution frequently even easier.
When things get gross
If you're like me, you have not been regularly cleaning your mat and have been using the same dirty, smelly mat for far too long. It gets to a point where things can't be ignored, and you may even reach a point where you wonder if you'll have to just throw the thing away. And the truth is, you just might.
Before you decide it's time for a new mat, you can try soaking your old one in the tub with some mild dish washing soap; just whatever you have next to your sink. You won't need much of it: about one tablespoon per gallon of water. You want to submerge your mat in this soapy water (best place to do this is a bathtub or large storage container) and let it sit for about 10 minutes.
After letting the mat soak, drain the bathtub, and take a microfiber towel to wipe down both sides of the mat. Once you've done that, you'll want to spend some time rinsing your mat with clean water. Once you feel like you've gotten all the soap off, let your mat hang for 24 hours. Again, avoid letting any sunlight hit your mat while it dries. Also make sure not to ring it out after taking it out of the tub; tempting though it may be, it is not good for the material.
Truly, a deep clean should not be a last ditch effort. Washing your yoga mat in the bat tub once every couple of months along with wiping it down after each use will keep your mat healthy and happy, giving it a much longer life to enjoy.