If your Sublayout Parameter values are coming through as null in your sublayout control, it may be because the standard values on the parameter template itself aren’t passed through*.
Standard Values for your parameters on the template that uses the sublayout will be fine, and the values will appear when used on any particular template. But if you are tearing your hair out wondering why your value is null, head on over to the template and set the value directly on its own standard values**. You should find the value appears.
* I am using Sitecore version 6.5.0
** or even the item in question, but that’s just messy