I think a lot of times we think "say what you mean" means speak your mind - and a lot of people use "honesty" as an excuse to just be mean. But these words are not a license to speak our mind freely wherever and whenever we feel like it - they are about being true to our word. They are about integrity. On the outermost level, they admonish us to be honest with ourselves and others about who we are. Most of us can do this, with a little effort.
But dig a little deeper, and they admonish us to also be honest with our intentions ("...and do what you say"). Sadly, many of us have a much harder time with this one. Why? Two reasons - we like to people please, and we don't like discomfort. We offer empty promises with the best intentions in the moment, but promises we know deep down we have no intention of keeping. This defies logic, because we are going to have to come clean at some point, but we do it anyway, hoping, I suppose, that the situation will just disappear. Sometimes it does, but often it doesn't. Either way, you have created a situation for disappointment and hurt, both of which could have been avoided had you just come out and said what it was you really wanted to say.
People respect people who are direct and who follow through on their word. We may not always be completely comfortable around them (that would be because we ourselves have some work to do in that department), but we can always appreciate that we never have to wonder about what they said and what it means.
Being direct and following through doesn't necessarily equal being compassionate. Add compassion to the mix, and people want to be around you. They know what they get when they're with you, and they like it. And let's face it - being liked is important to all of us. It's precisely why we do the dumb things we do to begin with - like making empty promises. Wouldn't it just be easier to be ourselves?
"Say what you mean and do what you say." That's all it really means - Be you, honestly, compassionately. Be you, and others will appreciate you. Stop struggling. Stop trying to be something else. Just be you. And be good with it.