I'm probably a bit younger than most of the moms on this site, so I could actually relate to your kids more. I can't think of much that you can do to fix the situation, but I can definately tell you what not to do. Don't yell at or hit your kids because it makes them scared of you (and you could permanently damage your relationship with them). Kids hate it when you tell them that you provide a roof over their heads, it makes them even more mad at you and makes them want to move out asap. Don't tell them everything you do for them, it doesn't go over very well when they're angry, it makes them not care and get annoyed. (Trust me, they appreciate what you do even though they might not say it). By the way, if you want them to appreciate the things you do for them, mention things when they're in a good mood. (Like if your shopping with them, ask them if they like the _____ you got them.) It'll eventually get them into the habit of saying thank you without you even saying anything about it, and they really will realize all the things you do for them. (Including providing a roof over their heads, it won't happen right away, but as they mature, they're going to realize that you do much more than just buy them things to play with or make them look nice).
Honestly though, out of all these suggestions, I agree with Julia the most.
The soap in the mouth and hot sauce things mentioned by some of the others might work. The only thing is I would want my kids to treat me nice just because they want to and it's the right thing to do; but making them put soap or hot sauce in their mouths might make them want to not say bad things to you just because of that, not because they don't want to say bad things about you.
I could suggest to be really upbeat and fun when they're being good, they notice it and want things to be like that more, so they'll try to be good. You could do anything from taking your daughters shopping and getting your son a new video game to making them their favorite food (or their favorite snack if you don't have much time on your hands). Another tip, don't be afraid to act goofy, they like it no matter what they say. I know I used to always tell my mom to stop whenever she was doing something I thought was dorky, but I really did like it, I had fun with it, and they're some of my best memories with her. (Just don't be too goofy in front of friends, because friends can sometimes be brats and put ideas in your kids heads which would ruin it.)
As you could probably tell, it's mostly from personal experience. The things that aren't are just what I wish my mom would have done.
Good luck and God bless.