Here are my OPKs from my most recent cycle (and I did successfully conceive this cycle!)
I started seeing the lines getting darker on CD13, so we BD that night. CD14 it was positive, but we didn't BD that night due to DH's back hurting too bad. CD15 the hormone was fading, but we BD that night (I forgot to write it on there before I took the pic). It's important to BD before the OPK turns positive, because then you have a good chance of the little spermies waiting there for the egg when it drops!