It was no "one" thing for us, (at least not one thing I can point to) but we did do:

1. weekly acupuncture for 4 months leading up to the BFP, including treatments on the day or around the time of the transfer; and
2. We'd had 6 single, day 2 embryo transfers and all were BFNs. On this cycle we transferred 2, 5-day embryos. Only one embryo stuck.

We had never tried transferring blasts, and I barely thought much of it, except that a friend of mine had done some research and told me that blasts are more successful because they are transferred into the uterus at the time that a 'naturally conceived' embryo would have arrived in the uterus. Day 2 embryos are transferred into the uterus, but 'naturally conceived' Day 2 embryos are still somewhere in the fallopian tube, so the environment in the uterus can sometimes not be "ready" for an embryo. Hope that made sense.

We were treated for male-factor infertility only, although I have polycystic ovaries (the polycysts, not the syndrome). Day 5 embryos were a good try for us because it's after Day 3 that the spermies really start to work hard - Day 2 embryos might look good, but the sperm hasn't started doing it's job yet. If an embryo lasts after Day 3 then they can tell that the sperm seems to be doing okay.

Good luck.