Most writing mistakes are widespread, but good writers just get better at spotting them. Some things you'll learn to watch for are:

• Overuse of jargon and business-speak, like "utilize" or "endeavor" instead of "use" or "try."
• Clichés are stale phrases that have lost their impact and novelty through overuse. If you are used to seeing it in print, don't use it.
• The passive voice. The subject of the sentence should be the person or thing taking action, not the thing being acted on. "Harry wrote this article," is better than "This article was written by Harry."
• Rambling. When you are not sure what you want to say, it is easy to phrase it in three or four different ways. A single concise sentence is generally better.

When you write something, you get very close to it. It is nearly impossible to distance yourself from it straight away to edit properly.

The longer you can leave a draft before editing, the better. Half an hour to two days is enough of a break to edit well. When you do edit, read your work out loud. You'll catch more problems and get a better feel for how everything flows.

Always remember that your job, writing to a friend, is to entertain. That can mean revelling in the odd pratfall. So, don’t just write about the mundane and pleasant things, try to give them the whole picture and make them feel something.

• You are extending respect and friendship. Write quickly, and preferably by hand.
• You’ll want to calibrate what you write to your relationship both with the recipient and with the deceased. Make it personal.
• If you knew the deceased well, sharing a couple of warm memories will let the recipient feel there’s a shared bond.
• If you didn’t know the deceased, you can make respectful reference to what you knew of them.
• Use tact. Don’t tell the recipient how they should be feeling.
• If you’re finding it hard to know what to say, you can acknowledge that; but don’t harp on it.
• Avoid operatic, or competitive, expressions of grief.
• Acknowledge, but don’t belabour, the grief and pain they feel.
• Focus on the individual excellence of the deceased rather than the consequences of the loss itself.
• Be tactful of their religion even if you don’t believe in it. 

Writing simply, and in an alive tone, using everyday words is more forthcoming and refreshing, than using unnatural sounding words. 

Write like a human.

If you read your written text aloud, you will realize where it can be worded better, or where to add or remove a comma. You can also let someone else read it.