Back escape from a seatbelt grip
This escape assumes you have allowed your opponent to take your back and hook your legs with his heels, but hasn’t yet gotten to a rear naked choke. His arms are clasped around your chest with his left arm under your left and his right arm over your right.
Immediately pin his left arm with your left and hug it tightly. (This has the additional benefit of making it difficult for him to choke you properly.) Push off your feet and put your body weight on his torso; bonus points here if you’re heavy. This is an unstable position, he will roll you off to the left or the right. Rolling to the right puts you in a bad position, so put your free right hand on the ground to stop this. Rolling to the right puts you in a better position because you can now pin his left arm with your body weight and free your own left arm. Do not lose control of the left arm; he was nice enough to give it to you for free, and you’ll just end up in a choke hold.
In fact, I’m going to canonize this as a maxim:
If your opponent was nice enough to give you control of one of their limbs, hold it tenaciously unless you’re trading it for something better.
(Similarly, here’s a starter list of your generalized “weapons” in grappling: Your limbs, your weight, your leverage, and your pre-existing knowledge and experience. Possibly in reverse order of importance. Your targets are your opponent’s neck, smaller joints, and stamina.)
Back to the notes, you’re now lying on your left side on top of your opponent’s left arm. Harass his right leg to loosen his hooked heels and to distract him as you patiently scoot your butt over his left thigh onto the ground. (This is not an elegant maneuver and you have to remember to keep his left arm under control the whole time.) Once your butt is on the ground you can turn your body and establish side control, from which you can either attack or move to a full mount.
Back escape from a rear naked choke
Same situation, with back control and his hooked heels controlling your legs, except that he has the choke hold in place. Let’s be honest, you’re probably fucked. But you might as well do something, right?
Drop your weight and scoot your butt forward, and try to protect your windpipe with your chin. He will probably just crank down on your jaw, which hurts quite a lot and may break it if he’s strong and ruthless. But you’ve bought yourself a couple of seconds.
Dammit, I can’t remember the rest. Well, here’s a video of a similar escape instead:
The fact is that you’re far more likely than not to end up in his guard or half guard, but that’s definitely a better place to be than his rear choke.
Rear naked choke from seatbelt grip
Back to the first position, except this time you have control from the back with your left arm underneath, right arm over, and your hooks in place. (He will probably try to mess with your grip and turn his hips and shoulders, so just hold tight.)
Grab his right hand with your left and feed it to your right hand, grabbing it by the ridge and turning it outward. Clear his left arm away from his body with your free left hand and grab your left shin to maintain your new position. Wait for the opportunity (that is, for him to forget himself and do something stupid) to initiate the choke with your left arm. Release his right hand, he will probably start pulling on your choking arm. Your free right arm can now go behind his head (grab your bicep) to complete the choke.
Basic warmups for pretty much anything
For twenty seconds each: jog, shuffle one way, shuffle the other way, criss-cross one way, criss-cross the other way, jog with high knees, jog while trying to kick your butt with your heels, jog backward, jog forward.
Ten times each: shrimp, forward roll, backward roll, frontward breakfall, sideward breakfall, backward breakfall.
Full list of stuff to practice:
.T-Position takedown, sitting down to mount
.Upa and roll
.Rear naked choke escape on the ground
.Back control escape on the ground
.Kimora from guard
.Guillotine from guard
.Side control to mount
..Pull legs down and stomp
BJJ-specific warmups (above)