At age 2 with my son, if he doesn't say please or thank you, he doesn't get what he is asking for. I remind and prompt with him. At age 8, I wouldn't prompt. I would make it a one-strike deal. If she demands to be taken to a friend's house, she doesn't go. I'm sure there are going to be obligations that come up that are unavoidable and you will still have to go, but the stuff that is fun for her that she misses out on will have the most impact. If she tells you to do something, I would completely ignore her until she used manners. At 8 yrs old, she can figure this out pretty quickly.
As for table manners, do you have the support of the rest of the family if you constantly enforce them, the entire time she is eating? If so, I would nag, nag, nag. And when the offense is serious enough, like the spitting of ham or licking her plate, she would leave the table. We had my husband's family, with his 6 yr old sister staying with us for about a year a few years ago, and her manners were atrocious. But no one else would correct her and it wasn't my place. All I could do was gently remind while trying not to overstep my bounds. Until one day when she kept showing me her food and dropping it out of her mouth because she knew it drove me crazy and I told her it was disgusting and she couldn't sit where I could see her anymore. Everybody thought it was ok or funny, except me and my husband. Then I felt bad and apologized, but it made for a stressful house, but I didn't have to look at her food anymore. She knew exactly what she was doing and stopped easily once called on it. If the rest of the family won't get mad when you correct her, I would just keep on top of it until she is tired of hearing you and just does it. I would also incorporate the reward chart, so she can see how nice it will be to get the rewards and you will be reinforcing the habit constantly when she doesn't do it on her own.
I am sure lots of moms will say there is a better approach and that nagging is harmful or something, but I just don't think you are asking too much for her to not slurp off her plate or be appreciative at age eight.
Added:
I'm certainly not saying to be mean, just consistent and firm. All kids need reminders of the little things and lots of practice. All kids forget to chew with their mouths closed, but you shouldn't have to remind not to blow bubbles or slurp off plates. I think she will catch on to that part pretty quickly. Another idea is- Have you told her it hurts your feelings to be ordered around? Maybe she doesn't even realize how it sounds, especially if this isn't the way you guys treat her. She might not have ever heard it to realize that it isn't nice, except for on the playground from bossy friends. She is definitely not too young to be reasoned with about how it makes you feel.