Add a placemark file to your post. Yes! You can post a placemark without using the share command in Google Earth. Use the method described in this post if you want to add a placemark to a reply.
Use folders to group your placemarks. Did you know you can put several placemarks in a folder and post the folder? You don't have to make a new post for every placemark you want to add. This is less work for you and makes it easier for others to download your placemarks. See this article in the user guide: Organizing places data.
Add paragraphs to your placemarks. Use html in your placemarks to make sure the content is displayed as you wish. Don't know html? Don't worry, just use some break or paragraph codes like this to add spacing.
Add pictures to your placemarks and posts. For placemarks, your pictures need to be hosted somewhere on the web. Most image hosting sites will give you the code you need for your placemark (example). Just copy and paste. To add images to your post, follow the instructions here. Remember to check the copyright before using images you find on the web. Using the advanced Google image search you can specify you want only images that are "labeled for reuse". More info about that here.
Want to get a little more creative in your placemarks? Check out the tools and help in this post.
You've posted your placemark and now you can't find it? See this explanation.
Members of the Google Earth Community on the former BBS platform may remember the old "active topics" option. This doesn't exist on the new platform, but there is a way to get a similar result, see this post. It's time consuming to set up, but just add your favourite forums, and once it's done you'll have easy access to all the posts in those forums (except "pinned" or "sticky" posts).
Click the the gear icon at the top right of any forum for more options and help.