Dynamically changing selection critieria in bound gridview

I am new to ASP.NET 2.0 and am redesigning some web pages developed in
classic ASP.

A common task was for me to accept input of text boxes and date controls to
provide a filtered list for the user.

I am looking to use a bound GridView but am having trouble. I tried
modifying the sqldatasource1.SelectCommand property, but it does not persist
when using paging.

I tried adding parameters, but I don't always have a set parameter list.

I need to conditionally change the SQL selection string based on input from
text boxes by the user. Could someone please point in the right direction?

I seems like every sample out there is based on one fixed parameter based on
a predefined drop down list value.

Thank you,

Tobin