Well some of his generals such as Hephaestion and Ptolemy grew up with Alexander and were loyal to him from the beginning including during the time when his father exiled him and Alexander's prospects for becoming king didn't look very good so they could be trusted because they were really childhood friends of Alexander. His main issues came from dealing with the older generals who were veterans from Philip's reign who grew resentful of Alexander's adoption of certain foreign customs such as aspects of Persian dress and allowing himself to be crowned as Pharaoh in Egypt. After uncovering a plot to kill him by a group of pages it was found that the son of Alexander's general Parmenion may have been involved and when the son was killed, Alexander had Parmenion killed both because it was suspected that he may have had knowledge of the plot and that he might seek revenge ofor the killing of his son (at this time Alexander had marched ahead with the army and left Parmenion behind with 20,000 men to guard the treasury and their supply line so if he wanted to, Parmenion could have cut off their supplies and used the treasury to revolt). Alexander also relied upon the force of his own personality and his generalship. He was greatly admired by his soldiers who were devoted to him so it would be unlikely that one of the generals could convince a large amount of the army to openly rebel. As for Antipater and other potential enemies at home(Athens) they were partially content with the massive amounts of riches that came back from Persia. Also throughout his campaigns Alexander was undefeated and people feared his wrath. He dealt with people who betrayed him severely and when there were time when provinces rebelled and fought bravely and successfully against the local Macedonian forces but would surrender once they found out Alexander himself was coming with an army. So Alexander secured loyalty from his generals and others with treasure, fear of his military strength, and through his own charisma.