A piece of advice Yrkoon9 gave worth repeating (over-under grab in the clinch).
In a thread long ago, possibly related to my amateur fight, Yrkoon9 gave a spot of advice on how to nail less-experienced fighters with a close-in hook during a clinch.
In the clinch, you have an over-under hold on your opponent and you're jockeying for space to get a good hit. Your love taps to the midsection aren't going well, and you can't get the space for a good uppercut.
Your underhooked arm isn't going to be much use, but the shoulder of that arm can be. In the clinch, you shoot your shoulder upwards and turn your hips to the inside to force the opponent's head up and back. When it gets there, you come back around with a hook with your "over" hand and nail him.
To recap - bump their head up and back with the shoulder of your arm that has the underhook on your opponent. Immediately throw a hook with the arm that had the overhook.
Another over-under technique involving knees:
You have an over-under grab on your opponent. He's keeping his head pressed on the shoulder so you can't do the above trick. He's staying pressed close so you don't have room to throw the knee.
With the hand that has the overhook on your opponent, drop it down to their waist so that the heel of the hand is on their hip. Shove the opponent's hips back to create room so that the knee on the same side as the overhooked arm can come up and nail them in the midsection.
Recap - Use the overhooking arm/hand to push the opponent's hip back and use the knee on the same side to nail them.