As a leader I will typically not go beyond a tanda unless we have previously agreed to be practice partners. In any case I am never bothered by a clear 'thank you". If that's not easy for you, you can always "need to get a drink of water", "rest your feet" etc. Chances are the leader will ask someone else and won't even notice that you are sitting there for a while. If he asks again later on, you can decide whether you are "still resting" or not. Ok, so your favorite leader asks you right afterwards, and your feet are suddenly ready to go again... so, you dance with the new leader... and it's not the end of the world if he sees it. You already danced a tanda with him and he should be happy about that...

If somebody seems particualrly sensitive and stops asking you after such an event, and you would in fact like to dance with him a tanda now and, then.... just ask him to dance the next time you feel like it.

I have stopped asking after being turned down a couple of consecutive times. Now, all of this is "just me" ... but take it as a data point.