Shlumpasana is a commonly practiced pose. It's versatile and can be practiced sitting or standing. I find it is most often practiced off the mat. Working at a desk or computer, eating, waiting in line, watching television or reading a book.
Quick side bar: I see it practiced in my very home by my almost college-aged son. I have a theory that the practice of "Advanced Shlumpasana" in teenage boys is caused by growth spurts. Specifically because when they grow so much, so fast, they are unable to properly engage or propriocept their expanding muscular posterior anatomy and gravity's effect on it. I "help" him by occasionally by placing two fingers between his lower scapula and gently pressing in to remind him to use his giant muscular expanse of ManBoy upper back to sit up straight. I will probably be doing this forever. Moving on...
Shlumpasana is the one pose I universally recommend students never, ever, never do. Or if they find themselves in it please come out immediately. Why is that? Because it is the worst pose ever. When either sitting or standing and here are just a few reasons why:
First: The shoulders come forward, caving the chest in and rounding the upper back. This squishes (compresses) your lungs so you can't breathe efficiently or entirely.
Second: Your spine moves into chronic flexion. The same muscles are continually contracted in the front body and stretched in the back body. As a result back muscles are lengthened/weakened and front muscles are shortened/tightened. It also increases pressure on the front of the spinal disks.
Third: Your internal organs are being squished.
Fourth: Your head and neck slide tend to slide forward. With the movement your head is no longer sitting directly on top of the spine. As a result, the neck muscles have to hold seven pounds of cantilevered bone and brain up against the force of gravity, instead of balancing the weight of your head on the neck bones as designed. Is it any wonder why your neck hurts?
Fifth: When sitting in a chair in Shlumpasana, all your weight channels down to your tail bone. The coccyx is a very fragile bone that can be easily bruised or worse, broken.
Sixth and in deperation: Vanity appeal. You just look terrible when you do it.
Here are a couple of things that you can try right away. Most importantly, it will increase your awareness to current habits which can help to create healthier ones.
When sitting or standing, keep the feet parallel and hip width or just wider than hip width. Parallel means the heels are a little further apart than the toes. This tiny adjustment ensures that the internal/external rotators of the thighs and hips are working equally.
Check in on your pelvis occasionally. Are you tilting forward or back at all? Bring it to level. Think of your pelvis like a bowl with hot soup in it. You don't want to spill too much soup out the back or front. This will help keep your lower back (lumbar spine specifically) in it's natural curve.
Do you have a tendency to throw a hip left or right when standing still? Quit it. Now. This will help your keep your lower back balanced by not elevating or depressing one side of the pelvis all the time. Better for the hip joint and lower back.
UNcross your legs while sitting. This keeps the pelvis level and...
...don't always lean into one armrest (if you have armrests on your chair). Full disclosure: I tend to be a left-leaner (not surprisingly), so I had to un-learn this posture. My trapezius and levator scapula thanked me, so will yours.
Is your desk/computer position correct? If not, correct it. I started to use two yoga blocks under my desk so my feet can reach the "floor" instead of being in continual "Barbie foot" resting on the chair legs.
Practice Tadasana in the grocery store, or any store, while waiting to check out.
I'm a big believer in basics. If you start in a properly aligned and supported place, you can move on to a myriad of other activities without difficulty. The above are, of course, suggestions and maybe bossy ones at that. Your body is unique in it's needs so be sure to honor those needs.
If you know someone who practices shlumpasana often, have a postural intervention. Friends don't let friends shlump.