Reduced blood supply (poor circulation) is probably the most common reason why your feet "fall asleep," although temporary nerve compression in the ankle or even near the knee can also lead to that "pins and needles" feeling. Temporary paresthesia of the foot — the medical name for it — is typically not something to worry about and usually pretty straightforward to remedy.
However, if your foot or feet are constantly asleep or numb, that could represent a more serious condition such as diabetes, so a medical assessment is a good idea.
Getting Rid of it Yourself
Change your leg position. In most cases, cutting off the circulation to your foot by crossing your legs is the reason it goes numb. The blood vessels around your knee can get compressed with leg crossing or other contorted positions. Furthermore, the nerves that innervate your foot muscles are positioned next to the blood vessels, so some nerve entrapment or compression is not unusual either. As such, simply change your position by uncrossing your legs so that your foot gets its proper blood supply and nerve input.
The foot that you cross over is typically the one that "falls asleep."
As the blood starts to flow properly into your foot again, it should feel a little warmer and somewhat prickly for a few minutes.
In addition to changing your leg position (if leg crossing caused the pins and needles in your feet), stand up from your chair in order to promote better circulation. When you stand up, you enlist the help of gravity, which helps pull the blood from the upper leg into the foot. Arteries have smooth muscle fibers that contract and push the blood down in correlation with your heart beating, but standing up might make the process occur a little quicker.
Moving your foot in all directions (circular motions for 15-20 seconds) will likely help with circulation also and reduce the numbness or pins and needles sensation a little quicker.
While standing, a light leg stretch (such as bending at the waist and trying to touch your toes) might help "wake up" your feet also.
Walk it off
After switching positions and un-kinking blood vessels and/or nerves of your lower leg, consider walking around to promote better circulation. The main caveat here is, make sure your have some feeling and normal strength in your foot otherwise you risk tripping or collapsing and causing an injury.
Once you change your position, pins and needles in the feet don't last for more than a few minutes.
Permanent foot damage can occur if blood flow is restricted and nerves are compressed for many hours.
Simply shaking your asleep foot may be a safer alternative to walking if you still feel significant numbness or pins and needles.
Wear shoes that fit properly.
Pins and needles and/or numbness of the foot is sometimes caused by poorly fitting shoes. Cramming your feet into a narrow shoe is not good for circulation or nerve flow and may cause your feet to fall asleep — particularly if you walk or stand a lot. As such, choose shoes that grip your heel tightly, support your arches, provide enough room to wiggle your toes, and are made of breathable materials (such as a leather insole).
Avoid wearing narrow-toed high heels.
If your foot symptoms are primarily on the top part of your foot, then try loosening your laces.
Get fitted for your shoes by a shoe salesperson later in the day because that's when your feet are at their largest, usually due to swelling and slight compression of your arches.
When at your desk at work, consider taking your shoes off so your feet are less constricted and can breathe.
Get a foot/leg massage.
Get a massage therapist or sympathetic friend to give you a foot and calf massage. Massage reduces muscle tension and promotes better blood flow. Start rubbing from the toes and work towards the calf so you help the venous blood return back to the heart. Allow the therapist (or friend) to go as deep as you can tolerate without wincing.
Always drink lots of water immediately following a massage in order to flush out inflammatory by-products and lactic acid from your body. Failure to do so might cause a headache or mild nausea.
Consider applying peppermint massage lotion to your feet also, as it will tingle and invigorate them in a good way.
Join a yoga class.
Yoga is an aspect of traditional Indian culture that encourages health by proper breathing, meditation and placing the body into various challenging poses. In addition to stimulating energy flow, the body poses act to stretch and strengthen your muscles and improve your overall posture. Increasing your flexibility, particularly in your legs, might prevent your feet from falling asleep when your cross your legs or put them in other contorted positions.
As a beginner, yoga poses might create some soreness in the muscles of your legs and other areas — it should fade away in a few days.
If certain yoga poses increase the pins and needles in your feet, then stop immediately and ask your instructor for feedback on your technique.