Hmm, I am going to post a slightly different reply here.
Even though they say not to go by the lines - I think that if the lines are getting progressively darker you are getting closer and closer to your LH surge. I used to use the Clearblue Digital OPKs when I was TTC Tristan. If I was nowhere near the LH surge, the control line would come up really dark and the test line would be none existent. Gradually the test line would get darker and darker until it was as dark (or darker) than the control line and those were the days I got a smiley face on the test! I always considered the smiley face the sign of the LH surge though - not before.
A good tip someone gave me was to start BDing a day or so before your expected LH surge as then the sperm has time to make their way up to meet the egg for when it is released. A lot of women wait until they get the positive before they BD and it can take a while for the sperm to get up and meet the egg (a lot of people think the sperm just shoot up there in seconds, but it can take up to a day or so) and by that time the egg might not be at its optimum condition. So I would say, BD a day or so before and when you get your positive, to try and 'cover all bases.'
Sorry for the rambling post - I hope it made sense!
Good luck getting that BFP!
xxx