Adding a Related Posts widget to your blog is probably the best way to make your visitors stay longer and visit more than one page.

First of all, make sure to backup your template. You never know what might go wrong.

Then go to Template -> Edit HTML and paste the following code into the page header:

<script type="text/javascript">//<![CDATA[var relatedTitles = new Array();var relatedTitlesNum = 0;var relatedUrls = new Array();function related_results_labels(json) { for (var i = 0; i < json.feed.entry.length; i++) { var entry = json.feed.entry[i]; relatedTitles[relatedTitlesNum] = entry.title.$t; for (var k = 0; k < entry.link.length; k++) { if (entry.link[k].rel == 'alternate') { relatedUrls[relatedTitlesNum] = entry.link[k].href; relatedTitlesNum++; break; } } }}function removeRelatedDuplicates() { <br /> var tmp = new Array(0); var tmp2 = new Array(0); for(var i = 0; i < relatedUrls.length; i++) { <br /> if(!contains(tmp, relatedUrls[i])) { tmp.length += 1; tmp[tmp.length - 1] = relatedUrls[i]; <br /> tmp2.length += 1; tmp2[tmp2.length - 1] = relatedTitles[i]; } } relatedTitles = tmp2; relatedUrls = tmp;}function contains(a, e) { for(var j = 0; j < a.length; j++) if (a[j]==e) return true; return false;}function printRelatedLabels() { var r = Math.floor((relatedTitles.length - 1) * Math.random()); var i = 0; document.write('<ul>'); while (i < relatedTitles.length && i < 20) { <br /> document.write('<li><a href="' + relatedUrls[r] + '">' + relatedTitles[r] + '</a></li>'); if (r < relatedTitles.length - 1) { r++; } else { r = 0; } i++; } document.write('</ul>');}//]]> <br /></script>


Save it, and then click the Expand Widget Template checkbox.

Scroll down and find this code :
<b:if cond='data:post.labels'> <data:postLabelsLabel/> <b:loop values='data:post.labels' var='label'> <a expr:href='data:label.url' rel='tag'><data:label.name/></a> <b:if cond='data:label.isLast != true"'> </b:if> </b:loop> </b:if>

Change the above code to:
<b:if cond='data:post.labels'> <data:postLabelsLabel/><b:loop values='data:post.labels' var='label'><a expr:href='data:label.url' rel='tag'><data:label.name/></a><b:if cond='data:label.isLast != "true"'>,</b:if><b:if cond='data:blog.pageType == "item"'><script expr:src='"/feeds/posts/default/-/" + data:label.name + "?alt=json-in-script&callback=related_results_labels&max-results=10"' type='text/javascript'/></b:if></b:loop></b:if> <br />

Save it, and now go to Template -> Page Elements and add a new HTML/Javascript widget.
Add the following code to it:
<script type="text/javascript"> removeRelatedDuplicates(); printRelatedLabels();</script>

Save it, and then click the Expand Widget Template checkbox.
Find the HTML widget just entered and place it after the main widget.
Then change code in the widget code to :
<b:widget id='HTML1' locked='false' title='Related Posts' type='HTML'><b:includable id='main'><b:if cond='data:blog.pageType == "item"'> <!-- only display title if it's non-empty --> <b:if cond='data:title != ""'> <h2 class='title'><data:title/></h2> </b:if> <div class='widget-content'> <data:content/> </div> <b:include name='quickedit'/> </b:if> </b:includable> </b:widget>

Its all done now.
Check out your Related Posts now!!