1. Use a Cache Plugin
2. Kill Spam
Use third party comment system
When things are critical or even if your blog has a high traffic,
you must adopt a third party comment system such as discuss, livefire or
Facebook/G+. When a reader/visiter submits a comment at your blog,
the system needs to insert the texts in your MYSQL database and
that creates a hell lot of load for CPU & if your DB is large or
if it is corrupted or if the system is getting multiple requests
simultaneously, things will go worse.
3. Stay Updated:
Never hesitate to update your Wordpress, plugins or themes.
On redular interval the developer find & fix the bug & release the
latest version of WordPress & its plugins/theme.
I strongly recommend you to stay updated with all the applications
/plugin/themes you are using at your Wordpress blog.
4. Remove unnecessary plugins
5. Reduce load on index.php file
Reduce the number of posts on homepage
Always use excerpts for displaying posts on homepage
Remove image files from homepage & unnecessary links
Would be better if you use a CDN service