Fortunately, both Jack and I love training sessions, so have had no probs with any training - other than recall, with a whistle or without. Among other things, he'll bring me the post and fetches my socks when it's time to go out (I wear sandals in the house).
So, number one is still recall training (he's getting better as he gets older, but not perfect). Jumping up at visitors is another tough nut to crack.
Running out of ideas but currently working on the 'Bang!' trick - good fun, but it's' early days - the penny hasn't really dropped with him yet...
If I could only get him to hoover up and mop the kitchen floor, we'd be working on that!