Assuming you've already been there done that with all the discussions, negotiations etc and this is a stand off...
to tidy his room : "In one hour I will go in there with a bin liner and anything on the floor will be put in there and put out for the bin men." -BUT YOU CAN'T MAKE IDLE THREATS!! Say it - do it! (psst, he can earn his stuff back painfully with good behaviour )
go in the bath: miss bath, straight to bed, no big deal. If he's really filthy, pin him down and scour him!
go to bed: Turn tv off, all lights off and sit there in silence and darkness. He'll soon get bored!
come to the dinner table: Easiest of the lot - no appearance at dinner table, food removed and no dinner for him! And no snack later because you feel sorry for him! Missing a meal means missing a meal.
Sometimes actually getting into discussions, debates and arguments with your child is the worst thing you can do. Sometimes you have to just act. Be the Boss! You aren't helpless if they won't cooperate.
I had a lock on the outside of their bedroom door. And I used the bugger! (Of course you stay there, silent!) I had the hoover (ds1 HATED the noise) I had the bin - so much ended up there! tv off, computer off, things removed, no dinner...they learn that life is simply easier when they obey the rules.
And you pick your battles too. Don't come down on everything. Choose those things that really matter. Talk first because obv agreement is the ideal, plus because they MUST be warned and be given the chance to cooperate, but be prepared to do whatever it takes. Stay calm, don't go to the same level as your child (by that I mean getting into an argument or acting like agemates) Don't back down. Never turn a no into a yes! etc etc.