Gracie Barra Bristol, (BJJ), Can Sönmez, Bristol, UK - 10/11/2011
This is right back into my comfort zone, as tonight, I'll be showing my favourite escape from mount, the foot drag. It is also quite simple, which is another reason I like it. You're in mount, your elbows in a good place for defence, down by their knees. For this escape to work, you need to have one of your legs out flat. You also need to get on your side (but watch out for technical mount): a slight bridging motion will help.
Next, wedge an elbow underneath their knee. You can either make a frame against their hips, or if you're concerned about your neck, adjust so that you can still pry your elbow under their knee while protecting your collar with your hands. Use a combination of your elbow and shrimping to shove their knee backwards, on your flat leg side.
Bring your other foot over both your flat leg and the leg they have next to it. That means you can use the heel of that foot to drag their leg over your flat leg. As soon as you get it over, lock half guard and shrimp towards their trapped leg. In half guard, you want to get onto your side as quickly as possible: if you stay flat on your back, you've already done their work for them, as they will want to flatten you out in order to pass half guard. If you're comfortable in half guard, you could stay there and work your attacks.
Alternatively, keep shrimping in the other direction, in order to free your other leg, just like you would with an elbow escape. It's also worth noting that some people, like Roy Dean, recommend just pinching your knees rather than fully triangling your legs around theirs, so that's worth trying too. To help recover full guard, you can also bring your arm across to their opposite shoulder, impeding their movement while aiding yours. Emily Kwok has a handy tip too: if their foot is tucked making it hard to drag, shove it out with your knee.
A very similar escape, which I don't use much, is the foot lift. Dean shows these two escapes in sequence on his awesome Blue Belt Requirements. The foot lift is for when they have some space underneath their in-step. People won't often do that, in my experience, but if they do, this time just step over your flat leg. Use your foot to hook underneath their instep and lift it over, then as before lock up half guard (your legs are already in position), or shrimp to recover full guard.
Make sure that you pay particular attention to shoving on their knee with this variation, as it is easier for them to slip free (though if that happens, you can always switch to the foot drag). With both escapes, it is important to get the knee of their trapped leg back behind your legs. If they still have their knee past your legs, it makes it much easier for them to move straight into a half guard pass, by driving their knee to the mat and sliding through.