Persisting data in a class

I have a custom class which I've declared an instance of on my page.
The problem I'm having is that the variables I've set values to in the custom class aren't getting saved, so I'm having to re-assign the values on the Page_Load() event. Ideally I'd like not to have to assign values once instead of on each postback.
I've tried using the ControlState but thats lead to horrendous results as its just a standard class which doesn't inherit any WebControls.
 
Any help would be greatly appreciated.
Thanks.
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