In HG/SS you can call Gym Leaders/Trainers at a certain time to rematch them. In D/P/P you have the Vs. Seeker to rematch trainers and can rematch Gym Leaders in the Survival Area. Is there anything like that in B/W?
Unfortunatly there is no way without Action replay or such code devices. The only thing that comes close is the PWT but that is in BW2.