If you have a boy, you can place cheerios or fruit loops in the toilet and have them aim at them. If you want them to sit, I always find bringing a story or something they enjoy into the bathroom will encourage the kids to sit for longer. Once they do use the potty, you can use a reward system such as stickers or M's to reinforce the behavior and make it so they want to go again. Also, now that the weather is warming up, take your kids out and let them run around with no diaper on. The experience of peeing will be that much more obvious and will probably help make them more aware of their bodies. Good luck!
I'm a preschool teacher and former nanny.