Ah, now I look at that again I realise I hadn't understood your button switches - I thought you had
them in the right place (between the base resistors of the NPN and PNP, thus switching them on).
You have to pull the NPN base high and the PNP base low simultaneously to switch the bridge, which
can either be done with a switch, or two opposite logic signals (assume 5V bridge), or an NPN
transistor replacing the switch, whose base is driven from one logic signal.
(as in this circuit http://www.mythic-beasts.com/~markt/bipolar-hbridge.png