This sample code should do what you're looking for:
Sub ClearTextBoxes()
Dim myForm As Control = Page.FindControl("form1")
Dim ctl As Control
For Each ctl In myForm.Controls
If ctl.GetType().ToString().Equals("System.Web.UI.WebControls.TextBox")
Then
CType(ctl, TextBox).Text = ""
End If
Next ctl
End Sub