Have you tried Vitex? When DH and I were TTC #2, DD was still nursing and I also hadn't seen a PPAF at all by that point (DD was about 10-11 months old at the time and nursing a lot still LOL). So I started on Vitex. I don't know if it was a coincidence or not, but I also got pregnant that month I started taking it.
I know for me in the past my cycles can be all over the place, my O dates are all over the place as well as my LP, sometimes it is long, sometimes it is super short. I've found the Vitex really helps to give my cycle the perfect balance for me. Here is a really good explanation of it, better than I could ever explain anyways!
http://www.susunweed.com/Article_Vitex_RedMoonHerbs.htm
Also like a PP mentioned, the B6 will really help. Try spreading your intake of B6 throughout the day if you can so your body will absorb as much as possible, as opposed to taking only what it needs at the time and flushing the rest out (which can happen if you take a good dose just once a day for example).
Also, False Unicorn Root as well has been shown to help. I haven't tried that yet, only because I haven't been able to find it in any health food store here where I live, so maybe it has to be ordered online.
Here is another great website that explains some beneficial herbs to assist in TTC and Fertility.
http://www.ovulation-calculator.com/pregnancy/herbs.htm
HTH!