Gracie Barra Bristol, (BJJ), Can Sönmez, Bristol, UK - 03/05/2012
It is common when on the back that you might find they manage to knock off one of your hooks, or perhaps you're struggling to establish that second hook. If that happens, in order to take the back fully, use the grip you have with your arms to put them on your side, towards your remaining hook. Come up a little on your elbow and pull your remaining hook up slightly. Bring that foot across their body to hook their other leg. You're looking to retain enough control that you can then reinsert your second hook, particularly if their reaction is to kick out that leg.
That does take a bit of flexibility, so it may not fit into everybody's game. There is another option, which Marcelo Garcia calls the 'hip extension'. This doesn't depend on flexibility. If they are blocking your second hook, cross your free foot over your hooking foot. Although crossing your feet if you had both hooks would be asking to get foot-locked, if you only have one hook, it means they can't properly apply pressure against your ankle. You can then thrust your hips forwards into them and pull with your seat belt grip.
The result should be that your partner is bent around and stretched out, so that they can no longer connect their knee and elbow to block your foot. That's your chance to quickly insert a second hook, before they can recover their defensive position. When doing the hip extension, don't forget to keep control of their lower leg with your first hook. Otherwise they can just pop over and escape.
Another simple option to keep in mind is when they've managed to clear one of your hooks, or it's slipping and you want to replace it. You might find that you can simply put the cleared hook foot on the floor (still keeping your knee tight) and bridge, to roll them back to the other side and re-establish that hook. Be careful though, as they are obviously going to react if you release a hook: you'll need good timing and close control.
Similarly, and again this is a Marcelo tactic, you can use your foot on your underhook side (so, the side on which you arm is threaded under their armpit) to hook behind their same side knee. Lift that high in the air, then dump then back towards your overhook side. This is particularly useful if they are trying to get back to the centre, bridge and press their weight into you, in order to start wriggling their shoulders to the mat and begin their escape.
Incidentally, Marshal Carper (one of the co-authors on Marcelo Garcia's latest book) did a handy video on maintaining the back Marcelo-style.