This is a small thing, perhaps, but nonetheless important:
use the serial comma!
All of us write, whether we like to consider ourselves "writers" or not. We write letters, we write memos at work, we write emails to our friends and colleagues. And when we do we are faced with a host of decisions that can, even if they seem insignificant, dramatically affect what we are trying to say. I've noticed that the serial comma has dropped out of favor in common usage, even to the extent that AP stories no longer use it. This is incomprehensible to me, as the serial comma is critical to maintaining continuity and meaning in a sentence. For example:
"I'll make us a dinner of grilled vegetables, tofu and red beans and rice."
"I'll make us a dinner of grilled vegetables, tofu, and red beans and rice."
In extreme examples such as this (when the third item in your list is compounded) it is absolutely critical to use the serial comma, as not using it results in a clumsy sentence that requires multiple readings. The point remains the same, however, even if the list is simple. Putting a comma before "and" clarifies the sentence and makes things measurably easier on the reader. Since writing is communication, your goal is to never have the wording obscure the meaning (unless it's intentional, and that's another conversation). So, use it. It matters.
While I'm at it, my recent fascination (obsession?) with typography has led me to understanding something else: you should only use one space after a period. The two space habit is a relic of the typewriter era, when fixed-width type demanded an extra space so that sentences didn't run together. Computers have eliminated that problem. Nowadays, adding an extra space (when typing in Microsoft Word or any other program) actually makes things worse: it creates awkward gaps, resulting in paragraphs that look choppy and inconsistent. It might be a hard habit to break, but you should put in the time to do it.