Having participated in a marital art (Brazilian jujitsu) that amounts to basically how to grab, take down/throw/trip/sweep someone, get on top of them, and choke, joint lock (or break) or beat someone up, and how to fight against that at every step, I'll mention a few things.
1. Do not fight back unless you, uh, know how! If I have to tell you this, ya don't. And even then if you're out numbered this amounts to "pulling guard" which is just putting your legs around the other person waist, and TIGHTLY squeezing yourself to them, and putting your arms around theirs or their torso or grabbing their upper arms. Naturally, if you don't know what you are doing it looks like dry humping or just nonsense. They're SUPPOSED to restrain you supine anyway, but if they insist on you turning your back I guess there isn't much you can do about it.
2. learn to fall? Simply going limp, holding your breath and closing your jaw (unless you like getting your tongue bitten off) can help fall without injury. Try to "roll down" instead of slam down on your hip or arm.
Also, if you're already on the ground they can't slam you on it, so keep that in mind!3. DO NOT PANIC. Stay calm, breathe, if there is pressure on your neck simply tense up the muscles at the front of your neck - someone could stand on you that way and you could still breathe. Keep your abdominal muscles slightly tense so you don't get the wind knocked out of you as easily. You tend to get the wind knocked out easiest if you are breathing in so be careful about it if you're being struck or someone likes to drop their weight on you via their hip, knee, or whatever to your back or chest.
4. If someone tries to cover your face turn your head quickly, and again, fight the urge to panic, it will only make it worse for you if they're looking for an excuse to hurt you and tire you out faster. Do not stop trying to breathe, keep focusing on that.
5. Go to a freaking BJJ class or something so you can put the damn guy trying to take you down into a crucifix or face lock :rofl: (not recommended...)
6. Try not to take it personally, or it will only start screwing with your mind. Sad to say, but thats kind of the point behind using restraints so damn much... submission to them. I'd say it amounts to rape without actually doing anything necessarily sexual

7. Avoid giving them reasons to restrain you in the first place. Ounce of prevention = pound of cure.