Building A Dynamic Sql Statement Into Stored Procedure

Hi i have a page whereby the user can make a search based on three things, they are a textbox(userName), dropdownlist(subcategoryID), and region (regionID). The user does not have to select all three, he or she can enter a name into the textbox alone and make the search or enter a name into the textbox and select a dropdownlist value, my question is how can i build this procedure, this is what another user suggested but i am having trouble;

ALTER PROCEDURE [dbo].[stream_UserFind]

( 

@userName varchar(100),

@subCategoryID INT,

@regionID INT

)
AS

declare @StaticStr nvarchar(5000)
set @StaticStr = 'SELECT DISTINCT SubCategories.subCategoryID, SubCategories.subCategoryName,
Users.userName ,UserSubCategories.userID
FROM Users INNER JOIN UserSubCategories ON Users.userID= UserSubCategories.userIDINNER JOIN
SubCategories ON UserSubCategories.subCategoryID = SubCategories.subCategoryID WHERE UserName like @UserName'

if(@subCategoryID <> 0)
 set @StaticStr = @StaticStr + ' and SubCategories.subCategoryID  = @subCategoryID '
if(@regionID <> 0)
 set @StaticStr = @StaticStr + ' and SubCategories.RegionId  = @regionID '

exec sp_executesql @StaticStr

)