Is there any battery meter on the SB-900? I don't know what's the battery level now but I shot over 700+ photos set it as fill flash, still can go on.
Anyway, I reached the "Thermal Cut-out" indicator like 5~6 times today

. Really panicked as the flash is not able to fire and the first time it happen when the bridegroom reached the moment to open the bride's room door

I pushed my ISO to 3200 but yet to get "acceptable" frame. Later on the flash regained and just in time to capture the moment he open the door~ phews! Thanks again for not happening during family shot and kissing moment.
How you guys managed to control the heat of the flashgun? Just turn off the "Thermal Cut-out" and shot only?
Anyway, it was a very good experience today. The situation changed where I become the OP because my friend is busy and the other photographer (friend too) absent. It's kind of important to lead them indirectly where then you'll be able to capture some good frames. Indeed flashgun bring photography to another level. I compared between my shots and my friend using the same body and same lens, that's whole lot more different! Thanks to everyone that helped me out for it with advise and sharing experience

It's good to hear you have a good head start in wedding photography. Keep up the good work, and you will be able to get a hang of it pretty fast.