This is a simple and obvious technique, but it took me a while to adopt it: You can do by asserting with NSAssert or NSCAssert for [NSThread isMainThread], just as you would assert any other condition.

Like this:

- (void)updateUI {
    NSAssert([NSThread isMainThread], @"Only update UI from main thread");
    // update UI
}

- (void)backgroundWork {
    NSAssert(![NSThread isMainThread], @"Only do work off main thread");
    // do work
}

With assertions like this in place, you’ll sort out what executes where in short order.

I wish I had done this to my code right away; it’s much simpler to get it right than to go back and fix it all later.