I don't have specific tactics to maintain focus. What I'm starting to realize is that there are days you really struggle to make something happen.These days suck.
Maybe you spent 4 hours listening to audio tracks and/or sound effects. Maybe the graphics are sub par. Maybe your game logic is flawed. Maybe you have too many layouts, too many event sheets and you know what? it just doesn't worth it.
But then you test for the millionth time the same layout. And suddenly it "clicks". Graphics, music and events all come together to form a level. Holy sh*t, I'm a game designer (not really but you catch my drift)!
To me these moments are the confidence boost I need to maintain my focus.