Ok.. a couple of basics on skirts for poser characters.  When you "conform" an item of clothing to the character it matches the bone structure inside the clothing figure to the bone structure inside the character you conform it too.  

Now, when the bones are bent those mesh areas of the clothing that are designated as part of that bone group will bend with the figure.  This works great for pants, shorts, and most shirts.  Not so with skirts.

When the figure moves forward or backwards eventually the edge of the skirt makes contact with the legs and causes "poke through".  There are a couple of ways of fixing this though, not to worry.

Most quality products put out by vendors use one of two methods, in fact some use both, to address this problem.  They will either add extra bones to the skirt allowing you to bend certain sections of it to allow it to conform to your pose, or they will add morphs that will allow you to make portions of the skirt come out, up, or move side to side.  Many use both to allow the user maximum control.

However if your working with the poser default characters odds are good your also working with very basic clothing that has no such capabilities built in.  That's ok, you still have a couple of options.

First, magnets. The allow you to deform part of the clothing to suit your needs.  There are a lot of really good tutorials on magnets out there, so I won't go into great detail on this unless a followup is needed.

Second, you can do what the vendors themselves do and add additional bones to the skirt in the setup room.  This is a bit more complicated of course and has a few more steps involved, but it gives your clothing the maximum adjustability with the greatest of ease.

Setup room tutorials are also available in abundance, I'm sure you can find one if this notion suits your fancy.

Hope that helps,
Stepdad