How can I make show/hide links for my posts?

Put this Javascript in the header of my template, as per the Help article

<script type="text/Javascript">
function expandcollapse (postid) {
whichpost = document.getElementById(postid);
if (whichpost.className=="postshown") {
whichpost.className="posthidden";
}
else {
whichpost.className="postshown";
}
}
</script>

Put these two styles in stylesheet, again from the article:

.posthidden {display:none}
.postshown {display:inline}

Now, instead of adding the rest of it to the template, just add the
rest to each post, in the HTML edit:

<span class="posthidden" id="id_for_this_article">

[[ hidden article text goes here ]]

</span>
<a href="javascript:expandcollapse('id_for_this_article')">
[+/-] show/hide this post</a>

The disadvantage is you have to remember to add those bits to any
article you want to have hidden text... the advantage is that you only
have to add it to those you want to have hidden text! You also have to
make up an article ID, and ensure it's different for each article you write.

[+/-] show/hide this post