Oh my goodness! I could have written this myself! My daughter is the same way, bossy as heck! I've been having discipline problems too.
I am definitely a spanker, but soon before she turned four it stopped working. Plus, she fights me when trying to get her into position and it turns out to be good workout for me but not much for discipline.
And yea, the sending them to their room doesn't work for anything but getting them out of our hair for us to calm down from being ticked off.
What I've found works wonders is a time-out. I put my daughter in a corner with no toys or decoration or anything around her, nothing to distract her. I keep her there for 5-10 minutes, depending on the infraction that got her there. Some would say that's excessive, but the shorter times didn't work.