Maintaining Posture in Closed Guard
If you're anything like me, you've gone through periods during your jiu jitsu path wherein you've dreaded being in your partner's closed guard. Posturing up is difficult, and it feels much more like you're trying to survive than you're in a neutral position, ready to open up your opponent's guard and pass. Here's a fundamental look at how to ultimately stave off this feeling forever (although there are bound to be closed guard standouts who will inevitably temporarily bring you back to your posture infancy for brief periods).
Hips and Hands
Maintaining good posture is often contrary to what intuition tells you. One example of this is how straight your back should (or shouldn't) be. Generally speaking, it is far better to push your hips forward and "hollow out" your stomach, ultimately looking something like a cat doing a stretch, than to have your back completely straight. With your back (and hips) "creeped out" in this manner, you can liberate your hands to stop any time your partner attempts to sit up. Obviously, if they can sit up, they will have a much more high percentage chance of breaking your posture, and bringing you into the arena of arm drags and overhooks.
Hands Free, Straight Back
This drill, while it may appear silly, is extremely useful in driving home the point that posture isn't really what we tend to think it is. Try a "hands free" posture drill with your partner, wherein your partner is allowed to pull you forward with their legs (they can only use their hands if they absolutely need to begin breaking your posture, EG if you are much bigger than they are). Keep your back completely straight, as one would assume "good posture" means, and see how that goes for you. "Beauty school posture", indeed.
Hips Hollowed, Same Drill
Now do the same drill with your partner, except this time, use what you know about hollowing your hips and stomach out. As your partner tries to use their legs to pull you forward and into their clutches, use your knees on their hamstrings as a place to send all of that force -- just focus on pushing your hips forward, and all of their pulling energy will be redirected (not magically, though it sometimes can seem this way) into their own hips, making it all but impossible to pull you forward. Note: you will still "lose" some of these battles too, as many folks are very strong at controlling posture with just their legs, but you should lose far, far fewer battles in this manner.
Biceps and Inside Control
Remember that maintaining distance is the real goal here, and sometimes "posture" can be a somewhat misleading term in that regard, since it does tend to conjure up "beauty school" posture (trying to walk in a straight line with a glass of water balanced on your head, for example). Sometimes your partner is able to control the distance anyway, ultimately pulling you forward into their domain. It's important to understand that this does not automatically mean that your posture is compromised. Instead, you can sometimes use this to your advantage by simply controlling your parnter's biceps, thus winning the "inside control" battle that is all-important. Once you're down there, you can grip your partner's lapels in order to effectively extend your reach, keeping their shoulders pinned to the mat, so you can stand up with a classic "logsplitter" guard opening.
Opening the Guard:
We Has a Confuse
Like many jiu jitsu techniques and fundamental movements, maintaining posture in the closed guard often doesn't look exactly the way we initially think it should. First, it is counterintuitive not to keep your back completely straight (and maybe even arched slightly), as this seems to be the right thing to do. Second, even if your partner has you close to them, it really doesn't have to mean that your posture is in any way compromised. Keep these exceptions in mind whenever you are examining other areas of jiu jitsu as well, as there are numerous examples of techniques that seem to defy common sense, until you examine them much more closely. As always, let me know if you are able to execute any of these techniques, and if any are confusing, feel free to reach out!