I was struggling with that just last year. I trained both my 4 and 2 yr old at the same time. A few things worked for us. First, I started giving my sons LOTS of control, since potty training is a control thing. They want it, they got it. Do you want to wear your red shirt or your blue shirt? Do you want to put on your underwear first or your shirt? Do you want orange juice or apple juice? Do you want to wear your jacket or just bring it with us? So even though this takes some time and effort on our part as they decide . . . you are filling up "their tank" by giving them choices all day long. When leaving the park/playground . . . say "do you want to leave right now or in 5 minutes"? Then give them the 4 minutes left, 2 minutes left, 1 minute left countdown and "time to go". If they argue, say "it was your choice to leave in 5 minutes, now lets go". So even though you cannot draw a direct connection between this and potty training, you won't believe the difference in the kids attitudes when they get some control, potty training is no longer such a control thing.
We set the kitchen timer (I actually have a small digital one that I can clip on my pocket and I can hear it where ever I am in the house. This is more to remind me (as I get side tracked and don't realize how quickly 30 minutes (or whatever period of time) goes by. Then I would not ask or demand . . . instead, I just say "Do big boys go pee-pee in their pants? Nnnnoooo. So if you need to go pee-pee, where do you go? That's right, in the potty. You are a big boy now, so if you need to go you just get up and run fast, fast, fast!" This is the only time running is allowed in the house - they love it. This is a reminder, even though you are not saying "go to the potty". You are mentioning it so they have to stop playing and focus on what you are saying. You are also inviting them to "show you" they can be a big boy and most kids love to impress us with their skills.
Since kids often like to play “teacher” I also set up a potty chart for myself. During the day I would often get their attention by saying “Mommy needs to go potty, do I go in my pants?” When they say “NNNNOOO” I would reply with, “Well then where should I go?” When they said “in the potty” I would ask them to show me where and what do I do when I am standing next to the potty (like pulling down pants, standing on step stool and sitting down). This got them really involved in the idea that potty doesn’t just come out, you get a warning feeling that should prompt them to do certain things (run, pants, sit, potty, pants, wash, reward). When I was done with the potty I would announce what a good girl I was and that I should get a sticker on my chart (and/or reward). Set the example and let them play teacher. This also prompts them to “go try” so they can get a sticker/reward too!
Don't get complacent with your praise. Sometimes instead of praising them (and stickers or other prizes) I would run around the house like a raving lunatic that just won the lottery and the boys would just die laughing at me and loved to go potty to get that reaction from me. Be enthusiastic and be consistent.