Hi,
Please refer to the below link for sample plugin.
http://msdn.microsoft.com/en-us/library/gg594416.aspx
The same code is for asynchronous plugin. You just need to change the settings in the plugin registration tool from synchronous to asynchrounous.
Please go to the following link to know more about plugin registration tool.
http://msdn.microsoft.com/en-us/library/hh237515%28v=crm.6%29.aspx
Thanks
If this post answers your question, please click "Mark As Answer" on the post and "Vote as Helpful" Thank you, DT