"Of course you can have a chocolate bar, baby. When you're finished that broccoli, we'll walk over to the store and buy one."
"You sure can go outside honey! Once we're done picking up your toys, we'll play on the swings together."
Keep direct negative replies at bay unless it's imperative such as in a potentially dangerous situation. A curbside "Mommy, can I cross the street by myself?" needs to be dealt with instantly and distinctly. Discussion and education can take place later.
Of course, you've got to be right on the ball otherwise it's quite possible that you may inadvertently agree to bungee-jumping directly the vacuuming is done. If you've erred, the back-up here to is belly laugh loudly (holding your sides for further effect), wipe your eyes and say: "My, my! I'm just being so silly today!"
If you're in a non-critical circumstance but still need to say "no" (such as the invariable plea to stay up a little longer), try going sideways: "On the weekend, you can stay up till 10:00, but only if you go to sleep right now".
By using "no" less often, it will be taken more seriously by the kids when you do say it. In the same way, your qualified yes gives the child what she wants ("yes, you can paint") as well as what you want ("as soon as we put away these puzzle pieces"). Tasks assigned with your proviso should be done together as much as possible, to keep your positive-negative positive, non?