It's not actually necessary to change your formation, as long as you mix your play up quite a bit according to how the game's going. Sometimes a different approach to the game or just a slight adjustment of player position (without changing formation) or tactics can do the trick. Think of it as similar to Football Manager.
And sometimes you're going to have to change your formation or get smashed. Being a proud crappy-team player, I've always used two formations & tactics, one to get a goal and another to hold on to it (5-4-1 usually). Surprisingly, I find it works even better with good teams. Well, I guess anything works with good teams.
