Yoga was (and is) traditionally practiced outdoors in India. But you wouldn't know it from the way it is usually sequestered inside yoga studios in the U.S. and other developed countries, most likely because its popularity began in urban centers, where finding outdoor practice spaces can be a challenge.
But in the last couple of years, many studios and individual teachers have begun bringing their asanas back outdoors again. And plenty of practitioners have taken their individual practices to their backyards, local beaches, parks and even mountaintops post-hike.
I do most of the yoga I do these days outdoors (well, at least in late spring, summer and autumn), as I value the time I spend enjoying the open air and sky, and I think that it's actually easier to focus on my practice when I'm immersed in nature. I'm lucky enough to live in coastal Connecticut — and I love to travel to ocean-surrounded locales — so I have sought out on-sand yoga, as well as practicing in the cool, restorative environment of the eastern woodlands. Here's what I've picked up:
You might not need a mat
I've found that it's easier for me to get footholds (and enjoy the totally different experience of digging my hands and feet into the sand for balance) if I do yoga on a sandy beach sans mat. I actually prefer practicing without a mat on lawns and grass too; just be sure to wear dark clothes so you don't get grass stains on your clothes. If I am on an earthen floor, it's great to have a mat to keep clean and protect your knees and feet from small pebbles or stones. But more often than not, when I practice outside, I don't use a mat at all. However, it's always a good idea to bring one, just in case; you can always use it as a pillow in savasana, and some sandy beaches are quite grainy, or might be wetter than you are comfortable with, in which case you can always roll out your mat.
Bring extra water
If you are used to getting through class without water, reconsider if you are practicing outside. It may very well be warmer or more humid outside compared to even an un-air-conditioned studio space. And take a larger bottle of agua than you might need; it's always best to have extra, especially if there is a hike or walk to the space you will be practicing.
Bring a hand towel
It's always a good idea to have a towel to clear off sweat, but outdoors, you may get damper (see above) and you also might want to wipe sand, earth or dew from your body.
Remember the wind
When we practice indoors, wind is not an element that we have to contend with. Outdoors, it can bring a wonderful sense of cool and refreshment (not to mention the beautiful aural enjoyment of the wind through trees' leaves). But it can also blow hair or any loose clothing around, and gust leaves or surf spray into your practice area. As long as you are prepared for this inevitability, you'll have a good time — it's usually the surprises that can throw us for a loop.
You might get interrupted
I find that my best yoga practices have taken place outdoors, but that doesn't mean that interruptions don't exist. Weather can change, friendly dogs can approach you with wagging tails, and tides might encroach on your practice space. Most of the time, it will be a great experience, but keep in mind that you are outside, and that when you do yoga outdoors, you are part of nature's dynamic system, which never stops moving.
I've never had a problem with insects (other than biting mosquitos) while doing anything outdoors, including yoga. I find that people who are the most anxious about bugs tend to attract them, so realizing that they are just part of the balance of the system — and have little to no interest in you — may help you focus on what you are doing, rather than what's flying around you. Some lavender oil rubbed on pulse points will keep most insects at bay, and keep scented hair products and moisturizers to a minimum so that local bugs won't be encouraged to come check you out and stick around.