Off spin is one of the few types of bowling I have a real battle plan against. Obviously this is only my plan, might not work for most.
I look to really smother the ball early doors (I'm quite tall with a bigish stride). Getting really over it and playing it solidly, letting the bowler know you could play him all day long should you want to.
Then I start playing the bowler on length. I don't really leave the crease unless the match really dictates it. I have a back foot punch through the off side that I use frequently to the back of a length balls. The really short ones you can put basically anywhere, my usual one being a big aerial pick up on the leg side.
The fuller stuff I look to milk as much as I can. Respect good length.
Also, use your crease. Nothing works better at making a good length ball a back of a length one than standing deep in the crease.
Or stand deep in the crease and step forward as he's bowling, try and upset his rhythm. Bare in mind all offy's want is a rhythm to just hit a length all day long. Anything you can do to upset that is useful (it doesn't always have to be tracking them!)
Just my crazy ramblings. I love playing off spin