Using lazy mouse and increasing roll distance will help you make straighter lines in zbrush. As for the sculpt, just keep sculpting! I recommend finding some reference to sculpt from, and over time as you sculpt more and more things you'll get better at coming up with your own ideas.
The best way to deal with this ... is to think of the final model game model as you sculpt, as opposed to considering the two steps as separate. And also, even starting on the lowpoly model *before* finishing the sculpt. The low is what goes in the game, therefore it's the most important.