I'm tring to create a custom button and custom link to open a Visualforce page, and have tried both:

/apex/pageName?param=value&otherparam=value2

/apex/nameSpace__pageName?param=value&otherparam=value2

Once packaged and deployed, I'm getting errors about the page not being found.  I've read suggestions that using URLFOR() with $Page.pageName is the correct way to do this, but I can't find any examples of using URLFOR with $Page and querystring parameters.