Using the disavow tool in Google is not a guarantee that your rankings will recover. First you have to make a deep analysis of the backlinks pointing to your site or your clients' sites. For this kind of research you will need special program which will find the backlinks which have NO VALUE pointing to you. These can be - comment backlinks, forum profiles backlinks, low quality backlinks with a big outbound links number (a lot of links pointing from a certain page).
The program I am using for discovering all these backlinks is SEO Spy Glass which can be found here - http://www.seo-spyglass.com/ - I am using the professional version which costs around $99. There is also an enterprise edition.
Also you can take advantage of these backlinks discovering tools:
SEO Moz - http://www.seomoz.org/rank-tracker
Majestic SEO - http://www.majesticseo.com
Ahrefs - http://ahrefs.com
Raven Tools - http://raventools.com
Once you know the 'low quality' backlinks - extract them to a text file (.txt format). Make sure to check the backlinks numerous times as you can disavow a link which can be helpful for your rankings! To extract only the URLs, not other texts I am using this simple and free program called Guru's URL Grabber - http://www.download3k.com/Internet/Email/Download-guru-s-URL-Grabber.html
Once all is done simply go to the disavow section of your site and upload the text file. You are now ready and just wait. Hopefully the results will come back! :-)
one very interesting discussion about the disavow tool on google's group forum - https://productforums.google.com/forum/#!topic/webmasters/X3ErEwg5yoE/discussion