I just competed in my 1st no-gi tournament & struggled with this exact same problem. Here is the advice that I was given afterwards. I've been drilling it as much as possible & really think it will help me in my next tournament on 9/24. If you want to see my matches I posted them about a week ago. In the match that I lost you can see me get to a great position - top mount & not know how to break a good bottom posture down. Needless to stay, it was the 1st thing I worked on when I got back to the gym.
Scenario: you are on top mount position. The guy on bottom has t-rexed up, locked hands, definitely keeping elbows & forearms glued to his sides. How does one gain access to this impregnable fortress off bullshit?
Step One: Bring his elbow to his ear & ear to his elbow.
Make your strong hand into a fist, keep your thumb knuckle facing forward. Wedge in your wrist-bone just behind the guys elbow. Support your wrist with your knee/thigh/low-crotch. It can take some work if he's concentrating on keeping his elbows tight to his sides but eventually you can usually work into this starting position. Once you get your wrist-bone into position, set your weight behind your wrist in preparation for pushing open the guys arm. If you push now, he will simply feel too strong to control so first you must also compromise his defensive posture from a 2nd angle. Brace the far side of his head while carefully positioning that elbow at a 90 degree angle. This angle will keep your arm safe so that the bottom player can't capture it using an over-hook with which he may gain leverage for a sweep.
You will have completely destroyed his defensive posture once you push his head & tricep towards a common middle ground about 1/2 way between their two starting positions (over his shoulder). You are simply collapsing the arm & head together. Strive for optimum leverage by keeping your strongest joints (hips/shoulders) tight & engaged. Once the arm & head are touching you can feed the hand that was pushing the tricep to now instead grab the head from the far side (behind his head). Use your head to now keep the arm smashed up out of position near the head.
Step 2: Transition or Submit the Fool!
1. You can dig your head in further to capture the tricep all jammed into his head to go for an arm-triangle choke from the top.
2. Or he may reach up to rescue the 'out of wack' arm & this gives you the chance to move up & sit tight to the armpits which will keep both arms out of posture. Great! Standard submissions from here.
3. Or you can transition to the S mount & should be able to maintain the arm that you already control up near his head (further reducing his ability to resist). You must concentrate on keeping your balance properly set while in S mount to avoid losing the position. Stay heavy on one side & keep one or more arms available to base when needed. Keep your knees tight to keep the bottom players shoulder posture destroyed. At this point, further transitions to side control, back mount, arm-triangle, key-locks, arm-bar, leg triangle are all possible.
Its working for me in practice - next weekend I find out if I can really apply it when I need it.
Good luck bro - I hope any of this helps!