"Get your act together" and "Clean up your act." They mean that a person (or group) should act more responsibly and stop making excuses. Let's say that X is angry because no one likes him. Let's say that X is lazy. X is rude. X is not clean. X says bad words. X is always late. A counselor might say, "Listen, X, you will never have any friends until you clean up your act." That is: Be courteous, be on time, wear clean clothes, speak correct language, etc.

Celia: These are two distinctly different expressions. They have different meanings. #1 "Get your own back". This means; "Protect Yourself." or "Look---Out---For---Number One. (You, rather than Me.)!" The common Acronym (abbreviation) for this in the United States is C.Y.A. It means; "Cover Your Ass." Another related expression that one sometimes hears is this one from a friend or associate; "I got your back." It means that a friend is protecting you or your interests. Your friend, if they Have-----Your----Back is protecting your reputation, your property, or your best welfare. That is referred to generally, as YOUR BACK. It's the part of you that you cannot see. In the USA, this is also generally associated with Watching---Your----Hind---End. In the USA, "you" is often referred to as your Bottom, the part of you that you sit on, as it is the "basic" part that is "you".

. #2 "Get Your Act Together." This is an imperative, or "command". It means Correct-----Your----Behavior. It is not a gentle expression generally. It may be spoken softly, but it is in fact, a very harsh criticism of a person. It is a strong suggestion that your behavior is flawed, your character needs much improvement, and as James so appropriately indicates, it shows a need for change. A similar imperative is the command to "Straighten Up" or "Straighten Up and Fly Right" or similar directives. The reference to ACT is associated with Human Psychology, in which it is generally understood that People have an ACT. It is a little too complex for a subject to enlarge upon, but it is sufficient to recognize that our ACT may need, from time to time, some corrective for your personal faults or our reluctance to show more wisdom and awareness about what is happening in the world around us. .

Thanks beforehand is same as 'Thanks in Advance'. Consider a sentence for example. "John, Can you bring me a glass of water ? Thanks beforehand!"