How to handle Auto Save Functionality in ASP.NET

[lolz r us]

I want to implement Auto Save functionality in \[code\]asp.net\[/code\] application, I had used Ajax Timer in my website,On \[code\]Timer1_Tick\[/code\] event I am calling \[code\]save_method()\[/code\], In save method \[code\]cache["User Id +data"]\[/code\] declaration is there to save data from this cache to \[code\]XML\[/code\] file on server,so for single user login it is working properly but when multiple users logs in data clash happens, so how to handle this functionality when multiple users accessing the same time...Help me.