I only have older nieces and nephews but here's what I know so far;
My middle sister and I have a boy and a girl 11 days apart in age. (they are both now 15 months or almost) My little girl has her moments, but is generally ok, for instance we can have sunday coffee at my parents house and she doesn't touch the breakables (and my mother collects breakable things, women!) B's cousin on the other hand is two hands full.
My middle sister's way of discipline is whining, 'No, no, NO, No" in that "carry on because you know I'm not getting up to do anything about it" tone. My husband and I do the firm No once thing, if you do it again we act, not by hitting but by taking away the thing or taking her away. There is no, I'm too tired to be bothered today mentality.
My middle sister has two boys, one 15 months one 4 years and they walk all over her. The oldest jumps on everything and my sister just throws her hands up in the air and says I don't know what to do anymore, then continues her No, no, no until I tell my husband we need to leave I can't take it anymore, the 4 year old does the same when at someone elses house but my sister adopted the 'it's your house so you do something if he bothers you' approach - never go that way! It just irritates the living crap out of everyone who is now responsible for disciplining your child.
My oldest sister has two girls, 5 and 3, I need a nap when they leave

she's a softy, today something is yes tomorrow she has PMS and it's NO. They are wild and out of control and mom and dad has this wait and see who of us gets irritated first before we act. Their father can be very strict, they know when they are pushing him too far and he acts on it. They have more respect for their father because of this just cause they know where they stand with him.
So from watching them I have learned no whining. Warning no just once then do something. When you start is up to you really because it does differ from one child to another. My LG for instance cries when I'm firm with her but as heartless as it sounds it's not going to turn me to mush because then she learns nothing. And when you discipline keep emotion out of it, as women we are constantly on a hormonal rollercoaster, so count to ten or 1000 before you discipline and think if the behavior is just bothering you because you are irritable, or your feelings were hurt etc