Break the problem/task down into independent pieces. Independent means that the piece can be unit tested. Finish the piece and its unit test before you stop cod

Break the problem/task down into independent pieces. Independent means that the piece can be unit tested. Finish the piece and its unit test before you stop coding. Design a good api/interface/signature etc for each piece to enable re-use. A good design and unit tests also allows you to refactor later on without blowing the whole thing up.