The first impression  a reader gets is the design of your blog. But it does not mean that your blog should be designed with full of graphics. You can simply impress your readers by using simple theme and arranging the layout of your blog in such a way that it looks impressive.

Sidebar plays an important role in this aspect, a second important part after content. Using the sidebar effectively is an art and you can learn this art simply visiting other blogs and see how they are using it.

Here are some ideas which can be applied to get the maximum use of sidebars.

1. Advertisements: Advertisements on the top of your sidebar give you maximum benefit and there are more chances that readers will click these advertisements. You can place different types of advertisements like adsense ads, image links to other blogs, paid image links, book ads etc according to the priority.
2. Recent Posts: Many bloggers place recent posts in the sidebar but I don’t recommend it. The reason is simple that the recent posts are already on front page/main page of your blog, so why use them. Some people will argue that if one comes on your blog through organic search, he may land on any page and if the recent posts are in the sidebar he can quickly look them. But my point is different. If you present good content to your reader and reader gets what he wants, he will definitely visit your homepage. Anyhow it is another choice for many bloggers.
3. Recent Comments: I prefer recent comments with commenters’ image in the sidebar to recent posts. It encourages other readers to leave a comment on your blog. For this purpose I am using “Get Recent Comments” which is one of the best plugin.
4. Popular Posts: You can add the popular posts to your sidebar on monthly, weekly or even daily basis. Some plugins are available which present popular posts according to the category. Again it is a good choice and it gives an idea to other readers that what is popular on this blog.
5. Tweets: As the popularity of twitter is increasing day by day, most bloggers want show  their tweets in the sidebar. Different plugins are available for this purpose or you can simple get the code from twitter site and embed it in the sidebar to get recent tweets.
6. Subscription Forms: Different types of subscription forms can be added to the sidebar like feedburner subscription, news submit form, contact form etc. The main purpose of these forms is to give an easy access to readers for subscription.
7. Polls: If you want to run a poll, it is strongly recommended that you add it to your sidebar so that maximum readers can participate in polling. A very good plugin is available for wordpress users to fulfill this purpose.
8. Tags and Categories: Tags and categories can also be added to the sidebar. However some bloggers prefer to add the categories to navigation menu in the header.
9. Search Box: The best place of search box is to put it into header. But it can also be placed in the sidebar but try to add on the top of sidebar above the advertisements or just below the advertisements.
10. Photos: Many bloggers add their flickr images to the sidebar and some in the footer.

There are also many other ways which can be used to fill up your sidebar. The best method to know is to visit different blogs and get the ideas how they are using the sidebar.