In my experience, a lot of new readers to a weblit serial won't start participating in the forums of a site until they've completely caught up with the story, largely because forums are often jam-packed with juicy spoilers that could ruin important reveals and plot twists. One way you can encourage new readers to migrate to the forums sooner is by fostering an environment that takes care to prevent unnecessary spoilers.

One great way to do that is to provide tools to your users to easily hide spoilers within a topic or reply. Here's a Drupal module that does just that:

Drupal -- Collapsible Text
Collapsible Text is a module for Drupal that allows you to cordon off portions of a text and hide them within collapsible blocks. Here's an example. Click the "spoiler" link below to expand the text:

The module is an input filter, meaning you can make it available to users -- in my case, I found it useful as a way to contain spoilers within forum discussions.

Collapsible Text is available for both Drupal 6 and Drupal 5.

BBCode Spoiler Tag
If you don't use Drupal, you're probably using some forum software like PHPBB, vBulletin, etc. Nearly all of these support BBCode, a sort of HTML-substitution markup language. Since BBcode is extensible, you can easily add a spoiler tag. There are quite a few of them, and they all work in different ways. I suggest a Google search for 'bbcode spoiler' and find the one that works best for your situation.