Yes, Dynamic clothing is the way to go when you are wanting to use sitting poses.  Either that or magnets or lots of post work in a graphic program after you have rendered.

Here are some tutorials on how to use dynamic clothing in the cloth room:

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2665209

http://www.philc.net/CC_sampleVideo.htm

http://www.poserfashion.net/howvic3dress1.htm

http://drgeep.com/p5/cr/cr.htm

Another nice thing about dynamic clothing is that you can easily use clothing on various figures.

Here are some places you can get some free dynamic clothing to try:

http://www.poserfashion.net/

http://www.renderosity.com/mod/freestuff/index.php?username=svdl

http://www.renderosity.com/mod/freestuff/index.php?username=mapps

http://www.renderosity.com/mod/freestuff/index.php?username=tabala

It's also possible to make conforming clothing into dynamic clothing:

http://www.renderosity.com/mod/tutorial/index.php?tutorial_id=868

And it's possible to take a conforming outfit and work with it in the cloth room so that parts of it act dynamic.

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2517340&page=1

Until I started working with dynamic clothing all of my figures were standing nearly upright because of the problems with not being able to get the dresses to fit over bent legs.

There is a small learning curve, but it's not hard and if you get stuck you can always ask someone here for help. And it's well worth taking the time to learn. 

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi