Try turning off Alaska... That alone saves a ton of room.
Also, you'll get a warning when downloading if there isn't enough free space to hold all the new stuff - If you download only the places you might fly before and immediately after the data expires, and then wait for the other data to expire before downloading the next set, you won't be using a ton of space to hold TWO sets (current/next) of data that you don't need.
Of course, it's much easier to not have to worry about that stuff, which is one reason why ForeFlight recommends at least a 32-gig iPad.