@Wana, I've been developing in DGDK for about 5 years now. In that time you just "pick up" little tips and tricks that make it easier to develop projects, like I made a set of classes over a couple of months that I can use with lots of projects to do all of the backend stuff, etc. to make the game quicker to develop.
Most of the polish (so far) was added using Photoshop. The interface is very finely-tuned to fit in with the style of the game, lots of indie-developed games don't bother with making the whole package look good which decreases immersion. Polish is one of the hardest gameplay "elements" to achieve, on-screen items have to bounce, rotate, fade, etc. very subtly to just make the game "feel" nicer and less stunted. You have to make every action have a well-defined result that the user can see. That only covers the surface of what polish is, but you get the picture.
What I can say to you is that starting smaller projects rather than tackling a full-blown zombie shooter. Practise is what you need. Also it takes a LOT of time to develop, you will have to sink quite a bit into each part of the game to make it fit in and look polished.
If I have the time, I *might* make a set of tutorials to create a decent-looking game with DGDK but that wholly depends on whether I have the time to make them (they would be pretty in-depth)
