SendMessage

how can i simulate a mouse click with "SendMessage" ?

I can never seem to get SendMessage to work for clicking a button. I use PostMessage and it works. Here is how I do it:

Public Declare Function PostMessage Lib "user32" Alias "PostMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
Public Const WM_LBUTTONDOWN = &H201
Public Const WM_LBUTTONUP = &H202

Public Sub ClickButton(hwnd As Long)
PostMessage hwnd, WM_LBUTTONDOWN, 0, 0
PostMessage hwnd, WM_LBUTTONUP, 0, 0
End Sub

Thanks