S.,
I've sold to them and to kid to kid in highland village many times. It is really easy. They don't take everything though. Nothing too warn, old, stained, or something they currently have enough of. They take some things out of season, but only the really good stuff.
They price things at about 25% of the retail cost, then they give you half. They price high end name brand things a little more than 25%, so you get a better return.
Bring in all your stuff folded and clean (no hangers).
They will go through everything and call you back that day to pick up the things they can't use.
You have two choices really; You can do as above and you sign something saying you will agree to the prices they quote for your things. Then when you come back they have the check waiting for you.
Or you can make an appointment to be there as they go through your things. This doesn't change the price they give, but if you think they are offering too little, you can decide not to sell that perticular item.
Kid to Kid gives you the option of taking cash or store credit. If you take store credit, I think they give you 5% more for your items.
You can also decide to donate the items they (at both places) don't take, that way you don't have to take anything back home with you.
You may also find it helpful to start with one store, get back your items they won't take, then take them to the other place. Every time I have done this, the second store ends up taking some of the discarded items.
Hope that helps!
A.