There aren't many situations when you will actually be singing acapella, and if you ever do, in a real situation, there's a good chance your pitch may wander a little - lots of singer's does. But who will notice anyway? Do it once in a while, but most of the time, you'll end up singing with an instrument or two, so that is a good way to practise. Maybe play chords under the voice rather than merely banging out the notes you're supposed to sing. As Laurence says, scales is only part of the regime. Chords of all sorts, not only major and minor triads, differing intervals in all keys, and learn to sight sing, giving yourself a start note only for a bit of acapella, then check after a couple of bars or so initially for pitch.
Another valuable thing to practise is singing along to an existing song, turn down the sound, and carry on singing. Not only does that check your pitch, which will most likely be fine, but also timing - the difficult part...