My friend swears by 'special alone time' with each child, especially if they're feeling jealous of the others - which is genius! It's so easy to get wrapped up in the day-to-day that you forget about the impact of giving individual attention, and I've found it really does make a huge difference.
Also, another trick she uses is actually taking the time to explain why we do certain things - for instance, why we can't buy every toy in the store or eat sweets all day long! It's something you likely do anyway, but she really hones in on the "why," making it relatable and age-appropriate. Her kids are much more understanding of boundaries because of this simple yet effective technique.
I've also noticed how important it is to get the timing right when introducing certain concepts or requests - almost like there's a parenting sweet spot! For example, my toddler is much more likely to agree to a haircut if I ask in the morning, and my older one is much more open to helping out with chores if I ask after school rather than at dinner time. It's amazing what a difference timing can make, and something I'm trying to pay more attention to.