One of the best parenting tips I've heard was actually disguised as a teaching strategy:  Don't Cave.   Let's say little Fifi is begging for more computer time, and you've already said she gets 10 more minutes.  8 minutes down the line, when you give Fifi  her 2-minute warning, she starts to whine for more time, and you cave and let her stay on for another half hour.   What's an extra half-hour on a rainy afternoon, anyway?

Fifi is now thinking, "Sucker!"  She has won the battle and the war.

It doesn't matter so much exactly where you draw the line on limits, but it's crucial to stick to what you say!  Kids can spot a wavering parent a mile away, and it only encourages them to whine even louder next time, knowing that you will eventually cave.  The more you stick to the original plan (even if it was rather randomly decided upon at the time), the less the child will try to change that 'no' to a 'yes'.