I believe that starting a piece like this is always the hardest part. I mean, I know what I'm thinking, but it's difficult to put that into a paragraph and not come across like I think I know everything. This is just going to be a reminder of the huge impact your words have on the life of your child(whether they are 3 or 30). Are you using your words to build your kid(s) up?
The first thing I'd like to talk about it is the tendency that parents(and people in general actually) have to notice and comment when their child has done wrong, but, when their child does something right, they may notice, but aren't as quick to comment. Are the only "talks" that you have with your child about things they've done wrong? Some way that they've tripped up? Or, do you have a lot of "talks" about the things they're doing right, with only a few disciplinary talks here and there?
The best way to get your child(and again this really applies to people in general) to improve in an area is to notice, and voice your appreciation when they do. If you see a strength your child has, encourage that in him. For example, if you have a child who likes to give, notice when he does, and voice it to him. How hard would it be, and how much time would it take to sit him down and say something like, "Bobby, I noticed that today that you gave almost all of your candy that Grandma gave you away to your sisters just to be nice, and I just wanted to let you know how proud of you I am for that. The Bible says that God loves a cheerful giver, and so when I see you willingly give things that you love away, it makes my heart very happy!" That may not be a very good example, but hopefully you get the idea. That will make him want to keep giving!
If you balance the talks with Bobby about his weaknesses(say, a bad temper) out with talks about his strengths, he'll be a lot more receptive when he's done something wrong. Because the problem is this: when Bobby is young, if the only time you sit Bobby down for a talk is when he has done something wrong, by the time he's a teen, he's going to have an association made in his subconscious brain: talk with mom/dad=negative feelings/failure to do what's right. Do you want your child to have that association?
The second thing I'd like to make you think about(don't laugh.) is playing the mature card and/or the "how old are you?!" card. First and foremost let me say this: I know it works. I know these cards work. Even I myself have used them. I mean, these work on anyone and everyone....not just your kids! But this is the thing: it is always used as an insult, to get the person to stop what they're doing. And insults are wrong. End of discussion. There is no excuse to use an insult. Ever.
So perhaps I should explain what exactly I mean by "the mature card" first. The mature card, is one that is most often used on someone whom you know is trying to be mature, or who thinks they're mature. And it's often used in a situation where, the person it's used on is most likely not acting mature. Saying things like, "Bobby, your temper is just uncontrollable! If you want to be mature then you had better rein that in!" or "Well that was mature!" or "If you want to be mature, you should do [whatever it is you're trying to get them to do]!" will not accomplish anything at all. You may as well say to them, "You're failing at the one thing that you're striving so hard to be!" Out of the moment of the instance in which said person is being immature(did you get all that?), it may be appropriate to approach them and say, "Bobby, I know you're trying hard to be mature. And there are so many things that you do, that show that maturity is growing in you. For example: (insert list of ways Bobby is being mature.) But something I think that you could work on is your temper and the way you just let it blow up at times." And then talk about that and find a resolution.
The "how old are you?!" card is another insulting card(even when it's used in jest). Yes, it works(sometimes...other times it has the opposite effect but is pretty much guaranteed to hurt whoever it is used on) because it gets their attention. This one is pretty common. Bobby lets his temper fly, and so you say, "Bobby, how old are you?! Five?!" You might as well say, "Bobby, you're losing your temper again, and that makes you a failure in my eyes." True, Bobby may not be acting correctly, and most likely has insulted or angered you, but that still does not give you an excuse to insult(whether Bobby is your son, friend, or husband). Also, this is often used in front of other people. So then not only are you insulting them, you're humiliating them as well.
I think it comes down to this: whatever it is that they're doing, wait until the moment has passed. Then talk with them. Do not talk down to them. Talk to them like they are as mature as they want to be. If you must say something in the moment, try and find a creative way of saying something, instead of using an insult to get them to comply. It's not easy. But try. Just try and see what happens.
I really have no idea if what I've said has made any sense at all. Please, don't be afraid to correct me if I'm wrong. But if I'm right, don't let it just go in one ear and the other. That's all folks.