Trying to screw up somebody's DI is relatively easy, since you have a couple of tricks that force them to guess.
When my opponent is near KO percents, I usually like to do the following stuff:
1) Start charging forward-smash, and then wait for it to end before d-throwing. You do this because instead of trying to get out of the grab, they DI; as a result, they sit there and wait while Nana fully charges up. Then d-throw for the extra percent and let the smash end on its own. This gets you more percent than normal, and against faster fallers--or when you're at the edge of the stage--this is more efficient than up-smash.
2) Do the same as above, but up-throw before you release so that you get a fully charged forward-smash in the opposite direction. Basically, what you just did is ensured that the opponent either guesses right and has a high chance of survival or guess completely wrong and screws himself over. Again, this works best against fast fallers because when they screw up DI, you can get a really early KO.
3) Between one and two, the only "safe" way to DI is up, because that gets you positive DI regardless of whether you send them left or right. Unfortunately for them, that doesn't really help against d-throw up-smash. So charge it a bit while the d-throw is going on and just let 'em have it. There are a sizable players who can react to whether you're f-smashing or up-smashing, but it gives them something to watch for and keeps them from seeing this next trick coming...
4) Down-throw with a MISSED down-smash. This is useful against fast fallers because you can chaingrab them if and only if they DI towards you. What better way to do that then to blatantly charge a KO move that--as far as they know--can ONLY send them the one direction? Their DI is obvious... except when you down-throw, release down-smash too late and just grab them again. You get the extra percent from a down-throw and confuse the hell out of them. So now if you charge D-smash again, they wonder which way to DI. And when you think they plan to DI away from you so you can't grab them again... just let go of the charged smash.
5) Fun tidbit; you CAN d-throw d-smash and, with the right timing, send certain characters backwards and behind you. My advice is that you don't use this simply because you might convince the opponent to DI away from you and then #4 won't work.
Between those tricks, you can pretty much scare an opponent into not wanting to DI at all. They have to guess, of course, but the funny part is that every DI option directly contradicts what they *should* be doing. This is excluding the different kinds of chaingrabs you have. When an opponent wants to DI towards you, d-throw d-air starts working again. When they want to get away, the smash is deadlier.
It's lots of fun, trust me.