I've really had a difficult time getting a character's dress to pose correctly when she is sitting. Most of the time I give up and either change the pose to standing or change the clothing to pants, which of course is causing me some limits.

I work almost exclusively with V4 and after conforming the dress to her, it seems I can place her in any pose and the arms and torso of the dress move just fine with her but not on her legs. I can't seem to set the correct parameters to get things moving in the right direction. On the dresses that come with handles, if I even touch one of those its like complete disaster, obviously I don't have a clue how they are supposed to be used.

I'm wondering if there are any tutorials or any tips anyone can provide to help me figure this out. It would be much appreciated.

Thanks,
stage

Dresses can be a pain to pose. Some vendors add morphs or handles to help in the posing of the sitting position but even then it can take alot of tweaking.
Magnets is a good way to get it to look right but I'm not an expert on magnets I'm afraid.
Regarding the handles. You can dial spin them to the desired position but it's probably best at first to just grap them & pull so you have an idea of how much effect they will have. To use them, select & then drag the mouse while keeping your finger on the button.
Sorry I can't help more.

Quote - I work almost exclusively with V4 and after conforming the dress to her, it seems I can place her in any pose and the arms and torso of the dress move just fine with her but not on her legs.

This is common, most people rig a dress with the entire skirt as the Hip.  I don't know if there is a good way to rig a skirt in Poser that includes groupings for the legs all the way down to the feet.  Generally the vendor leaves you with some posing handles to help shift the skirt around but it's difficult to get a conforming skirt to truly act like real cloth would.

For flatting the bottom when sitting, one or more magnets at the dress's hip would be the way I'd do it (probably just one).

One way I've done conforming skirts...and it does take some (sometimes a lot) experimenting to get it working just right is as follows:

Group everything for the skirt part to the hip bone.

Then, in the setup room, add a series of bones parented to the hip bone that further control the skirt movements. For short skirts, you'd only need one level of extra bones placed. For longer skirts you can add however many levels down from the first set of bones added in. It would in effect like having the extra leg bones in, but you don't need to worry about them being "mesh grouped" so to speak.

If you need an example of a simple base set up, I have this freebie here where I used these extra skirt bones instead of morphs.

It's not an exact science, but have had decent results setting things up this way. I need to go back to some of my earlier freebies and update them to work better with this. 

Another option if you're using P5+ -- export your dress/skirt out as an obj file with welding turned on, import back into Poser, parent to your figure, and then run a cloth sim. Can get varied results...but is another option available.

One important bit of info if you take that approach - and it is a good approach, that is how most of the best commercial conforming skirts are rigged - you have the Hip bone, and it is associated with the "hip" polygon group.  If you attach a bone to the Hip (call it "RightHandle" for example), and do not associate it with any polygon group, then that bone will deform the "hip" polygon group, which is Realmling's point.  However if you connect a child bone to RightHandle, and the new child bone is also not associated with a polygon group, then the new child bone will not deform anything.  Can be really annoying when you don't realize this is what's going on.

Another option is to run the skirt through the Cloth Room, it's far easier than re-rigging the outfit, and can produce some fairly realistic results. No need to export/import anything either.

If you have one of the Morphing Fantasy Dresses from DAZ, try it out with that first, works good even for standing poses.