Hi Cath and Denise, I don't know how you are prediciting ovulation etc but my main 2 suggestions (that you may very well be doing already) are:
-multiple inseminations over a number of days
-most importantly is to be inseminating prior to ovulation as ideally there are swimmers waiting around when the egg is released. So if you are using OPK's this means doing the first insemination prior to a positive OPK which is fine if your cycles and ovulation days are reasonably regular but can be tricky otherwise

Hope this may be of some use and good luck. Other things which I am sure you are alredy doing includes position, time from ejaculation to insemination and appropriate temp keeping of the little swimmers. hope you get a BFP soon, Girldoc