when you create an army, you start an army cohesion bar. its lower right corner. the longer your army is together the shorter that bar gets. when it gets too low your army disbands. best thing to do is just start armies with your companion parties, travel around with them or stay at a castle or city till they disband. everyday, at least at the beginning, you gain 1 point in leadership skill. also, use lords you have spoken to and have relation with, the higher relation you have the less influence it takes to get them
another little trick i like to do is get as many prisoners as i can hold, run around until they are ready to join my army but keep them as prisoners. call my companions for an army, let them join up with me , then go sack a castle. right as the siege camp is built, right before you attack, go make all those prisoners part of your army/party. that way you get an extra 50-60 troops basically for free and there is no time for them to desert