I'm going pretty good with the current strategy in my game (for full disclosure, I'm using the Zoroastrian flavour mod, but it doesn't add anything to the early game that should be factored in much).
I've had a bit of luck on my side, The Abbasids were always too busy with someone else to ever deal with me, except for joining a couple wars where I was the aggressor, which I rushed to a conclusion before their stacks could reach me, one way or another (a few white peace results, but one complete victory before they could make a difference).
I went for a simple divide-and-conquer strategy with my conquering. Like it or not, when you only have about 2-4 duchies worth of territory, owning it all in a huge demesne doesn't count for squat. You don't have enough men to take on more than one of your neighbors at any one time, even with your starting troops. In this situation it's too much trouble to assassinate or tyrant your way to owning all the choice lands. One the first things you should do is get your house in order by marrying off all the women you have (but any that are really good should be kept for concubines), to ensure there are plenty of Zoroastrian nobles for the future. Good traits are just as likely to pop up completely at random as the bad ones. And you can always prune the family trees of your empire every couple generations. Dump all your honorary titles on your vassals if you have to, to get their opinion of you up a bit, as well. Because you're to have to go to war early on.
I attacked Tabaristan. It has two Zoroastrian ruled counties, meaning that those two lords will be very loyal to you once they're yours, giving you the lion's share of their troops. I next went after Khorason, as a war was already going on between the neighboring Muslim nations, and I was able to snatch away the Satrapy before the Abbasid's doomstacks reached me. If they don't push Eastwards too much, then you can make out like a bandit against your neighbors, even with the added risk of their involvement in any Holy Wars. The next bit of territory was Samarkand. It cuts off Khiva from their other Muslim neighbors, and leaves them too weak to be a serious threat ever again. I did much the same to the Saffarids next when I took Hormuz, splitting them in two and leaving Birjand cut off and an easy later conquest after you consolidate your current position.
At this point if the Abbasids haven't turned in your direction you should only be left with a small collection of Muslim states too small individually to be a threat, and too busy trying to eat each other to mount an effective defence. Once you've conquered everything (or close to it) East of Khozistan, all the way to the north shore of the Caspian Sea, you're generally strong enough to take on the Abbasids in a defensive war, smashing up their armies one at a time before they can mass up enough to crush you.
One important detail though, going back to what I said about not bothering with a huge demesne early on, is training your first few generations of heir to be a great military leader. Stewardship can be left to your spouse to in high numbers. They'll usually be a close relative anyway, so if you didn't see that they were properly trained the blame is all yours. I can't tell you the amount of times I've marched my armies to success with a Martial 18+ ruler in control.
