Just say YES!

The average child hears "No" a gazillion times a day - eventually they very reasonably tune it out.
Try saying stuff like -
Yes, after you have finished that job.
Yes, when you are 18.
Convince me. Right now I feel that would be unwise - convince me otherwise.

Or you can try finding out what they really want behind the thing they have asked for...
eg - sometimes we eat because we are emotionally empty - maybe we could have a cuddle instead?

Have a loving day!
K