This isn't necessarily a grammar or spelling tip, this is more for writing than grammar itself, but a little rule that I learned during my novel-writing days that I still do today is when writing dialogue or taking down large amounts of exposition, or creating essays, is to avoid repeating words in the next sentence. Like:
John noticed the gun on the table. Curious, he reached over to pick the gun up, but the guard slammed his hand on the table.
"No," he said, returning to his position by the door, "the gun is not for you."
Would become:
John noticed the gun on the table. Curious, he reached over to pick the firearm up, but the guard slammed his hand on the desk.
"No," he said, returning to his position by the door, "the pistol is not for you".
It just feels less awkward that way.