The fundamental reason why the first crusade was a success was a disjointed Muslim foe. The Fatamids and the Turks were too busy fighting each other over territory, among other disputes.

The turks though did try and repel the crusaders, as most famously seen in the recapture of Jerusalem in 1099. But generally speaking, the majority of warfare during the first crusades were either skirmishes or seige warfare.

Chroniclers of the time ranged the crusader army to be between 100-600, 000 strong. Though it is most likely to be around the 60,000 mark (Thomas Asbridge - The First crusade). Even then the siege of Antioch, the crusaders suffered heavily from disease and famine, so by the time they reached Jerusalem, their numbers dwindled. To this extent, open battle was not really a wise move, and simply got lucky that the defenders of jerusalem gave up before their turk relief army arrived.

Beyond the first crusade, the encastellation of the levant was the primary reason why they survived so long. The crusaders knew that being in the levant, they would never be able to out match the Muslims, despite the fact they did on numerous occasions fight the Muslims in open combat e.g. The battle of Hattin - in which it allowed the Muslims to recapture Jerusalem.

The Templars were quick to build a plethora of fortifications, as were the hospitallers a bit later. Read Benjamin Kedar and Ronnie Ellenblums work regarding this, since they have spent entire career researching and digging these fortifications.

To sum up really - the first crusaders were just l lucky they didn't find a unified Muslim force. If they had done, it would have been pretty one sided for the Muslims. Beyond the occupation, the Frank's, crusaders etc had to relie on fortications and alliances to make sure they remained there intact.