How to change my sitting posture?

I realized that even sitting on the same chair... some people have a different sitting posture than others? Is there a way to choose my own sitting posture?

Hi Twinkboy, and welcome to SL!

What you're seeing are "animations" or "poses." These are little programs that make your avatar do certain things, and can be installed into objects, so that when you "sit" on an object, your avatar twists itself around. You can also keep these animations or poses in your Inventory, and "Play Inworld" to make your avatar do these things. Or, you can do what many people do, and get what's called an "Animation Overrider (AO)".

This AO changes how your avatar stands, walks, sits, etc by installing purchased animations into an object that you then "Wear." (Most often, it's a tiny object that ends up embedded in your upper thigh so that no one can see it.)

You can purchase animations or poses at many, many places across the grid. Search on "animations" and pop around to a bunch of different places to try out different looks for your avatar. If you decide to go the AO route, and most people do, feel free to ask back here how to set it up. Lots of people get frustrated the first time they try it.

If you have an animation overrider like ZHAO II, you can load it with new animations that you buy at places liske Animation Warehouse and others. I need to do this for sitting too (when I'm human) because the default sits are rather wooden I think. The instructions in ZHAO II are really very easy and it is free.

Of course a lot of furniture comes with it's own pose ball sits, but you probably know all that. Some of these you can touch the furniture to change poses depending on how it's scripted.

[Edit: Or what Oryx said ^]