My two cents:
If the idea of sounding like you're rambling is your biggest fear, then try to outline your posts. Think of the topic you want to work on, then drill it down into different aspects that should be covered. From there, drill it down even further, to illustrate the finer points of each aspect.
Then get a friend (or some other person) with some proofreading ability to look it over. Ask them to pay particular attention to tone and to the structure. Ask them if the post looks like stream-of-consciousness writing, or if it seems well-organized.
One thing to consider, though, is that it's okay if you get off track every once in a while. Unless your intent is to write professionally, your readers will (usually) recognize that you're a human being, and are willing to forgive the occasional transgression.
Most importantly, write the way you're most comfortable. As time goes on, your skills will improve for the better as long as you're making the effort to steer them in that direction.