Ok, I know that they're expensive but, if you can afford it, the Clearblue monitor may be helpful to those women that have a hard time with OPKs.  The monitor doesn't just detect your LH surge.  It also detects your estrogen which goes up 2-3 days before your LH.  It gives you more of a notice than OPKs alone.  Plus, you don't have to read it yourself....the monitor displays the results on the screen.  I don't use one but I did in the past and it gave me a few days notice.  I loved mine!