No matter how big or small your website is, especially a forum your posts and replies should always be engaging. This is because no matter the size of the website you will have a period of inactivity. I've faced it a decent amount of times on my largest website, and will continue to face it from time to time. One way to decrease the inactivity is to ensure that every post you make is engaging, as well as every reply you make is engaging. If you're wondering what makes a post engaging, it's simply what you can do to make people talk more. For example, welcoming messages. It's better to have the user post an introduction than it is to send an automated message. Though automated messages are good, you should probably have an introduction section as well. This will show a more personal response to the user. And posting your replies to their post should always be engaging, even if their intro isn't. For example "Hello, I'm new." Believe it or not, I get this a lot. Depending on your forum or website will depend on your response, but always ask them some questions and gear it towards your forum. We'll use Webmaster.College for example. A reply for that would be "Hello user1, and welcome to the forums! What are some things you're interested in that brought you to the forum?" this will engage the user by allowing him to further reply, as well as both show you're interested on a personal level and get him thinking about the content on your forum. When making new topics engaging, you should check out this post right here which is an article specifically to new topics and content.