I think the keys are to create a more defensible and economic geogragphy. To do this you need to create control of the inland cities that reduces bases of attack form the Muslims.
First Crusade:
A. Have Antioch turned over to the Byzantines, and have the troops that were based their go to Edessa. Now you have a strong Allie in Byz, and enough troops to defend the north (Edessa)
2nd Crusade :
Consolidate and listen to Byz advise in crossing Anatolia. he combined crusaders would have brought badly needed reinforcements and perhaps could have captured Damascus or better yet Aleppo. Aleppo would have further consolidated the northern part of the kingdom, brought a rich city into the KOJ, and eliminated a major attack point of the Muslims, as well as destroyed Zengi power.
3rd Crusade :
With the elimination of the Zengi threat the KOJ would not have lost Jerusalem, so who knows if the forces brought forth would have been as massive. Lets say it was. Now if you dont have Barbarossa go swimming and drown, then you have a massive German army marching south along the coast , and Kings Richard and Philip at Jerusalem. The really only main targets would be Damascus and Egypt. With the forces arrayed each would be realistic objectives and obtainable.
So you now have defensible borders, tremendous wealth, and inland areas protected from raiding which would allow for future population growth and increasing wealth. This is an area which could have survived well past the 1400's. The threats could only be mounted from Mosul or Bagdad. You do have the Mongols to deal with in the future.
can I late add the qualifier it has to own Jerusalem 
