Cleaning up your WordPress site will have a myriad of benefits, i.e. backups will be smaller and run faster with less errors, greater stability …
Deactivate and Delete Unused Plugins
This is a no brainer but most of us just leave old plugins around until they become too irritating. Some plugins will cleanup behind themselves, so check for uninstall options whilst the plugin is activated. Make notes of those that don’t and you can cleanup up the options table too.
Replacing old outdated and no longer supported plugins with newer updated versions is good practice too.
Reducing the Size of your WordPress Options table
Removing Transients
You can do this manually via phpMyAdmin which is explained on the site below
http://wpfreaks.com/easily-reduce-clean-size-options-table-wordpress/
You can view transienets in phpMyAdmin with the query
SELECT * FROM `wp_options` WHERE `option_name` LIKE ('_transient_%')
And delete transients with
DELETE FROM `wp_options` WHERE `option_name` LIKE ('_transient_%')
Remember to change the wp_options table to whatever you have updated your prefix to and look to your security if your table is still prefixed with wp_
Plugins
Transient Cleaner – this is apparently the way transient removal will be implemented in WordPress at some point.
WordPress Clean Options – the plugin is old, no updates in 2 years but it still seems to work. It can certainly give you some insight into how much ‘junk’ is in your options table.