"'No' is one of the few words that can never be misunderstood."
But let your statment be 'yes, yes' or 'no,no'.
Matthew 5:37a (NASB)
From God's Little Instruction Book
The problem is some people lack self confidence and their 'no' does not always mean no. Creates much confusion, especially for children. And creates resentments in the person who is not consistent in their 'no's'. They get upset because they are not honored. So, let our 'no's' be 'no's'.
"You have to decide what your highest priorities are and have the courage--pleasantly, smilingly, nonapologetically--to say 'no' to other things. And the way you do that is by having a bigger 'yes' burning inside. The enemy of the 'best' is often the 'good.'"
--Stephen Covey