The first and perhaps most important thing we should bear in mind is that when all is said and done, from a political and military perspective the Crusades were a massive failure. The First Crusade was successful enough that European leaders were able to scratch out kingdoms which included such cities as Jerusalem, Acre, Bethlehem, and Antioch. After that, though, everything went downhill.
The Kingdom of Jerusalem would endure in one form or another for several hundred years but it was always in a precarious position. It was based upon a long, narrow strip of land with no natural barriers and whose population was never entirely conquered. Continual reinforcements from Europe were required but not always forthcoming (and those that tried didn't always live to see Jerusalem).
The whole of its population was around 250,000 concentrated in costal cities like Ascalon, Jaffa, Haifa, Tripoli, Beirut, Tyre, and Acre. These Crusaders were outnumbered by a native population around 5 to 1 they were allowed to govern themselves for the most part and they were generally content with their Christian masters, but they were never actually conquered, merely subdued.
The military position of the Crusaders was maintained largely by a complex network of strong fortifications and castles. All along the coast the Crusaders had fortresses in sight of one another, thus allowing quick communication over large distances and the mobilization of forces relatively quickly.
Frankly, people liked the idea of Christians ruling the Holy Land, but they weren't very interested in marching off to defend it. The numbers of knights and rulers willing to spend blood and money in defense of Jerusalem or Antioch was very small, especially in light of the fact that Europe was almost never united itself. Everyone always had to worry about their neighbors. Those who left had to worry that neighbors would encroach upon their territory while they weren't around to defend it. Those who stayed behind had to worry that those on the Crusade would grow too much in power and prestige.
One of the things which helped prevent the Crusades from being successful was this constant bickering and infighting. There was of course plenty of that among Muslim leaders as well, but in the end the divisions among European Christians were worse and caused more problems when it came to mounting effective military campaigns in the East. Even El Cid, Spanish hero of the Reconquista, just as often fought for Muslim leaders as he did against them.
Aside from the reconquest of the Iberian peninsula and the recapture of some islands in the Mediterranean, there are only two things we can point to which might qualify as military or political successes of the Crusades. First, the capture of Constantinople by Muslims was probably delayed. Without the intervention of Western Europe, it's likely that Constantinople would have fallen much sooner than 1453 and a divided Europe would have been greatly threatened. Pushing back Islam may have helped preserve a Christian Europe.
Second, although the Crusaders were ultimately defeated and pushed back into Europe, Islam was weakened in the process. This not only helped delay the capture of Constantinople but also helped make Islam an easier target for the Mongols riding in from the East. The Mongols eventually converted to Islam, but before that happened they shattered the Muslim world, and that too helped protect Europe in the long run.