If you are logged into the forum,click on your own username at the bottom of the forum under "currently active users" (it will always be the first name). That will take you to your own user profile. On the lower left-hand side click the link that says "find all posts by..."
If you are looking at one of your posts, and would like to find the others, click your own username in the post, and select "find more posts by" from the drop-down.
Another method, is to click search, click advanced search, and select search by username. Enter your own name in the text box, select all forums, and it will return all your posts (up to 500 of them). To find more than the last 500 posts, you must filter by date.