One of the biggest problems new skaters have is stopping (on purpose ^_^). In order to get WFTDA, you only need to do T-stops and plow stops, but sometimes something just isn't right. Here are some ideas that can help:
Whatever you do, don't drag your foot behind you, perched only on the inner edge, with your skate bouncing! This is a good way to put too much resonating force on your ankle and break it, or to have someone in the pack behind you step on it, and break it!
So there are two ways to do a T-stop. The basic way is if these feet are skating up. You have your weight on the right foot, and you start digging your left foot into the ground. Ideally, you will dig with all four wheels equally. Technically, you are digging your outer wheels harder. While this is what they will actually be testing you on, it is also good to practice with these feet going left, so that you are standing on your left foot and digging your right foot -beside- you. Why this variation? So that you don't have your foot trailing behind you, again allowing for someone to step on it! As with everything, don't forget to work both with your right foot in front, and with your left foot in front. You never know what direction you are facing when you have to stop.
Plow Stop (or "how low can you go?):
I've been reading many people say that the slicker floor/wheel combination you have, the easier it is to plow stop. I can't really confirm nor deny this since I've only skated on slippy sport court with 88A's since I managed to do it right, but I can definitely vouch for spreading those legs! Then again, I am inhumanely flexible, so YMMV.
But you do want to make sure you are dropping your center of gravity by lowering your ass (not your shoulders!) and focusing on digging in your inner front wheels. As with T-Stops, make sure all eight wheels are flat on the floor so you don't flip your ankle. Once you can manage the plow, you can start transitioning to the
This is a variation of the Plow, where you have managed to turn your body 90˚ in either direction and are placing the majority of your weight on your front foot. This video has a good idea as to how to transition from one to the other, by placing marks/sticks on the floor and trying to imitate them with your feet. Though, unlike ice, we need to stay a lot lower!
This is a very effective way to stop, especially if you want to pick up and go in the opposite direction! You start by skating forward, then you turn around 180˚ and dig in your toestops. The more advanced skaters can do this by jump turning, but a more deconstructed approach would be the "book"approach:
Skate forward. Lift the foot from the direction you want to turn and "open the book", placing it on the floor facing backwards. Then lift your front foot and "close the book", placing it next to the other foot, also facing backwards. Lift your ankles and dig in your toes. It is good to note that your feet don't need to end next to each other, and that staggering your stance (one foot farther forward than the other) can offer greater balance.
I don't really like this chicks videos because she talks really slow, but at least she has good form and information. Here she goes through T, Plow and Turn stops.