Tending to misuse comma
I don't think the commas are the problem; instead, the problem is probably knowing the best places to put sentence boundaries.
As RegDwight says, there is no actual grammatical rule that says a sentence must only be a certain maximum/minimum length or contain a maximum number of clauses, or else it is wrong. So, there can be certain circumstances where a very long sentence is the best kind of sentence for something. However, I find that this is a reasonable rule of thumb for constructing a sentence (a "rule of thumb" is something that is helpful, but shouldn't be considered unbreakable):
- Don't have more than two independent clauses in a single sentence. (And remember that having and, but, etc. doesn't always mean you have two independent clauses.)
Look at these three examples (independent clauses are in brackets):
[Having a good memory, I remembered the way to get home], but [I have had a poor sense of direction for as long as I can remember].
[I am tired and hungry in the afternoon], and [because of this, I make a lot more mistakes at my job].
[I try not to create run-on sentences], [but it is difficult to avoid doing so], [and my friends always make fun of me for it].
In (1-2), we only have two independent clauses, and I think most people would agree that these sentences do not run on too long. In (3), there are three independent clauses, which breaks the rule of thumb; some people might say that this sentence is not too long, and some might say it is. I break this rule of thumb all the time — the point is that if you follow this rule, you have a reasonable amount of expressive freedom without having to worry if your sentence might be too long.
If you want to show a connection between two complete sentences, but using a comma violates the rule of thumb, you can use the semi-colon. For example, you could repair (3) by doing the following:
I try not to create run-on sentences, but it is difficult to avoid doing so; my friends always make fun of me for it.
At the risk of sounding overly obvious, how about separating thoughts by using periods? Just try breaking up an overly long sentence into a few shorter ones.
That being said, having long sentences is not "improper" in and of itself. Yes, sometimes it is a matter of comprehensibility, but above all it is a matter of style. Having extremely short sentences can be weird, too:
He has done well. But a lot remains. At least I think so. You might disagree. And that's okay.
But again, that is not "wrong", either. It's a question of style. There are many respected writers who are famous for their long sentences, or their love for commas, semicolons, parentheses or dashes. There are also many respected writers who are famous for short sentences, or for not touching a particular punctuation mark with a ten-foot pole.