I just found out by searching at google jpfitness on "flat butt" and found exercises that will correct the flat butt. Any exercise that works the Glutes, but glute activation always comes first and it teaches you and has you feel what glute firing feels like. For the Glute Maximus, Glute Bridges or Bird Dogs. For Glute Medius, Side Lying Clams or you could do a laying on all 4 clams (I feel this one better than the side lying ones. I dont feel the side lying ones at all) like a male dog goes pee. For the exercises, exercises such as deadlift & its variations (including single leg ones), Lunges, Squats, Step Ups/downs, Leg Lifts etc.
One person (a physical therapist on that site) said the easiest way to learn to activate your glutes is the "Prone Butt Squeeze" (10 seconds hold per side or whatever amount of time you want). After you get the hang of that, then you know what to feel for in glute bridges and other exercises too. First what should be done before even starting glute activation is stretching the hip flexors/rectis femoris and then before glute medius activation (x band walks or clams), to stretch the pirformis first.
By the way to the poster of this thread - thanks for making this thread because it makes me think about my flat butt I have from sitting a lot. Now I am going to do something about it and this encourages me to start! Search menu is my friend. I rather try to search for answers first before actually asking them and waiting for answers if they are ever answered=) I just decided that I will stick to bodyweight work right now until I can get my glutes to fire correctly and get into memory the firing feeling. I am hoping even without using any weight that I can get rid of the flat butt with just the activation work and bodyweight exercises. I really want to not do many weightlifting exercises at home until I get back into the gym in the middle of feb when school starts up again. Bodyweight is safe and I cannot hurt myself with it. Its good to be able to do everything bodyweight really well before doing it with weight or even doing it as a warm up exercise for that movement before using weights. It will make me feel better hopefully, stronger, and know how to feel all the muscles that I am working with for that movement. I am also hoping this gets rid of my anterior pelvic tilt too.