When I teach a yoga or Pilates class, many of my students complain that they're in pain. No, it's not their muscles burning from working too hard - it's their poor bones that dig into the ground. Ouch!
Regular mats are fairly thin, and offer no cushion for your bones when they're pressing into the mat. Some mats are thick but too cushy, and your body parts end up sinking into them - not so good for balancing poses.
I've found the Queen of Mats - Manduka . The Black Mat  is my absolute fave. It's thicker than regular mats, but it's also dense, which is what sets it apart. That firmness makes this mat ideal for any postures - even headstand.
This mat cushions your spine, hip bones, knees, elbows, and feet without you having to fold it in half (like other mats). The Black Mat  is a bit expensive, $79.50, but completely worth it because this will be the last mat you'll ever need to buy. You can buy them in 2 lengths - 71" or 85" if you're a tall yogini.
Fit's Tips: This mat is also heavier than typical mats, weighing 7 lbs, so if you want a firm mat that's a bit lighter for when you go on those yoga retreats, check out the PurpleLite Travel Mat . It weighs 4 lbs, is $57, yet is still durable enough to last a lifetime.