-- Reading online is not the same as reading from a piece of paper. It's harder on the eyes to read every word of a long block of text on a Web page - and I don't know about you, but every time I try to read something online that requires me to scroll down to see more of the page, I lose my place at least once.
The solution? Break your writing up into more easily digestible bits. People prefer to scan online, so get to the point, make it, and get out. Keep your paragraphs as short as possible, and have as few of them as possible. Use bullets for clarity if it makes sense.
-- OK, this isn't a tip or a trick, just something to be aware of: misuse of homonyms, or words that sound alike but that have different meanings - and spellings.
When you're writing your site, make sure you haven't mistaken "your" for "you're." Likewise, look out for "its" and "it's." And the list goes on. "There," "their," and "they're" are three more words commonly mistaken for each other.
If you're in doubt, do what the pros do: consult your dictionary. Mine's never been wrong yet.
-- Wondering about the tone your site should have? Here's my advice: write in your own voice. Write just as if you were talking out loud to your audience. It will be more readable, and chances are, your visitors will enjoy reading what you've written because it's unique.