It's not a debate

Parenting is NOT a debate. It's not you running for presidency and having to face your opponent (your child) and have a debate to prove who is the better candidate. 

Within reason YOU are the boss of your household, not your children. I say within reason because you don't want to be so strict or bossy that your children resent you, you don't want to be like the lady from the movie American Crime. You don't want to be an asshole or a bitch.

At the same time you need to be the driving force behind the direction you want your children to take.
Let's say you want your children to put their dirty clothes in the wash and they start complaining, whinging, protesting. That's where you say "It's not a debate". At no point is this open for discussion where they get to verbally derail you and twist the scene into going their way.

Or your child doesn't finish their lunch and a few minutes later they're asking you for more (different) food and you tell them they're getting nothing because they didn't finish their lunch. If they begin to plead or argue with you you say "It's not a debate". Under no circumstances is it ok for them to badger you into changing your mind. It's not a negotiation. 

Even if you say no initially then feel your no was too hasty NEVER EVER change your mind. It's a sing of weakness and you don't want your children jumping on that weak moment and using it to play against you. Especially if they start nagging. You don't want to reinforce that nagging is ok or that it works.

Recently one of my children asked their parent if they could go out out with them. The parent said no because it was nearly bed time. They nagged and repeated the question, adding please, please, please. The parent stood their ground until right at the last minute, almost out the door, they changed their mind and said ok. I jumped in and said no to my child and reminded my partner that they should never change they mind. Even more so when the child has pestered them into changing their mind. It's a bad message to send to children. They left and I looked like the bad guy. I'd rather that than the child getting away with nagging the parent and winning.

Try not to nag or repeat yourself either. Let you children know from the get go that you don't want to repeat yourself. Do this by saying "I don't want to repeat myself", "I've said it once already" or use counting to 3. Not 5, not 10... 3! Children don't need a great deal of opportunity (time) to continue doing or not doing what you've asked them to. The point to counting to three is you want them to STOP and do as you've asked, now... not later.
Let them know what your intentions are by clearly stating I'm going to count to three and then I'm going to [insert consequence here]. At the end of counting to 3 follow through with the consequence straight away.