Patience. If you don't have it or can't learn it, don't bother with kids.
I can only speak from the experience of having a 5 1/2 month old that is fairly happy as far as babies go. She doesn't cry a lot at all and is mostly "giggles" when she's awake. The trick is, we give her lots of loving attention and routine. She's well pampered. You can't spoil a baby. We play with her when she wants playtime and we lay her down for a nap when she wants it. We try to not to linger too long in a restaurant or while out shopping and manage to avoid public crankiness for the most part. We don't ignore her when she cries. We don't leave her in a room some where to "cry it out". Because we pamper her at this age, she'll healthily respect and trust us more when she's older.
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As far as older kids go, these are some rules my wife and I have talked about.
Always give reasons for why you are telling them something and always answer their questions. Take every chance you get to satisfy their curiosity. For example, never say, "just because I said so" when they whine about having to go to bed. Give them an honest reason and they'll respect you more for it and build up a natural urge to always ask questions when no reason (or good reason) is given. Always answer the "why" questions. And don't lame out and say "god did it" or "you'll know when you're older" or "you're too young to understand". That's not an answer and it'll stunt their natural urge to be curious. If you don't know the answer to their question, turn it into a learning experience and go to the computer with them or open a book and make it a fun experience to find out. You both will learn something.
Never tune out your kids. I know others who do this and their kids walk all over them and act like little pricks in public and at home. If you can't be the master and ignore them for long periods of time, they won't respect you when you do try to put your foot down.
Never sit your kids in front of the TV for too long. TV should be an occasional experience, not a baby sitter. They'll never learn how to entertain themselves and be content without it if all they ever do is watch TV all day. It can stunt their natural curiosity and creativity.
At dinner time, eat at the table. Turn that meal into a family occasion where you can share food, talk about the day, and relax. Don't have Mac and Cheese every night either. Make decent, meal-worthy food. Eat different things all the time so they grow up not being picky. It's more work, but it's worth it for them and you. Don't eat in front of the TV or in the car.
Let them play outside and get dirty. Let them explore.
Don't rely too much on bribes to get them to cooperate. Bribing too much leads them to expect a reward for cooperation every time. Instead of bribing, try giving them a honest reason. Hopefully, if you've done much of the above, you wont have to bribe much.
