The sidebar of a blog (or website) is one of the most important areas. Most blogs ignore and have the standard widgets in place. You will see the latest articles, latest comments and maybe a tag cloud with a few ads. This is a waste of prime real estate that could instead be used for monetization or content promotion. Here are a basics to follow on your sidebar:
Replace the latest articles widget with something that highlights content more appropriate to the page. It could be top related posts or top articles from the same category. Ideally the list should have between five and ten items and no more. Draw attention to the content with thumbnail images of the listed posts. For sites with less content, a site-wide most popular posts or latest posts would suffice.
The area on top of the fold is the best place for ads. Usually a single ad is what visitors will tolerate without ignoring your sidebar. Use a single ad, above the fold on the sidebar to get the most exposure.
Don’t use a tag cloud on the sidebar. This is something that was used at the start of blogging. Nobody clicks on the tags anymore and this is simply a waste of space.
Always have a category listing on the sidebar. Limit it to top level categories and encourage users to click through with post counts for each with parenthesis. As always smaller sites can just show the list until they have enough content to put totals