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A simple trick is to just have an outline of what it is you intend to write about. Digression usually occurs because you're making up the connections in your head as you go and get lost in a tangent. Know where you intend all your points to occur in the piece and you'll never give the impression you're digressing.
Jhouser's tip is very good. Once you have used the outline to help you write your piece, you can double-check its focus by looking at a few things:
Does the composition establish a clear focus somewhere near the beginning?
If so, does the first sentence in each paragraph connect to your main idea and build on it in a clear way?
If so, do the other sentences each paragraph maintain the focus established by the first sentence of that paragraph?