The approach of manually tracking each Subscription might become unwieldy if there are multiple Subscriptions that need to be tracked in the same Activity. In this case, a CompositeDisposable might come in very handy. It's a class that can keep references to multiple Disposables and then unsubscribes from all of them at the same time.

So, consider this case:

Disposable disposable1 = Observable.interval(1, TimeUnit.SECONDS)
        .subscribe();

Disposable disposable2 = Observable.interval(1, TimeUnit.SECONDS)
        .subscribe();

Disposable disposable3 = Observable.interval(1, TimeUnit.SECONDS)
        .subscribe();

Here, we have three Disposables that we would normally need to unsubscribe from with three calls to .dispose(). By introducing a CompositeDisposable:

private CompositeDisposable disposable;

disposable = new CompositeDisposable();

Disposable disposable1 = Observable.interval(1, TimeUnit.SECONDS)
        .subscribe();
[...]

We can assign all the Disposables to CompositeDisposable with this:

disposable.addAll(
        disposable1,
        disposable2,
        disposable3
);

Then, the onDestroy() block will look like before:

@Override
protected void onDestroy() {
    if (disposable != null) {
        disposable.dispose();
    }
    super.onDestroy();
}

Then, there will be no need for a separate .dispose() call to each Disposable.