We had different rewards for a wee or a poo in the potty. My DS loves blowing out candles. So it was one candle for a wee, and two for a poo - Soon got the message that a poo was something special to do. Soon stopped asking for candles for a wee, but carried on asking for candles a bit longer for poos. We never had any poos on the floor (and only one in the pants ever!). He just got the idea of doing both, and loved the applause, not to mention the TWO CANDLES.
Of course he tried to talk us into two candles each time he weed, but we stuck to the "rule", and this really motivated him to try for TWO CANDLES. He has never withheld a poo even when constipated (which can be a problem in some toddlers). It was a bit inconvenient and nutty taking candles everywhere, but it worked like a charm.
Oh and when we were out we never took a potty, but just told him there was no potty and he'd have to do it in the big toilet (and made visits there when we thought he had gone a long time without using the loo - at first these were under protect, but he soon got the idea that even if he didn't ask for it, we might still sit on a loo "just in case"). We did initially keep the nappy on him when going out (too chicken to risk an accident). But even though he was not keen on the "big toilet", he very soon started asking for it when going out and soon refused to go out in a nappy. Not had any accidents since going nappyless (thank goodness).