Getting the right mix of people in your band is essential. First and foremost, they have to have the right skills. Second, they need to get along with everyone. This does not mean to say that you should pick people who are plain boring or have no personality. Rather, it means that they have to be people who will handle pressure, commitments and relationships with maturity. This is essential for the long term success of any band.
If you look around the world and back through time you will see that 99.99 % of all bands break up. Sometimes, this is because a band member feels that he/she can make on their own. But most of the time the reason why established bands break up is that personalities and egos clash and creative differences become irreconcilable differences. This is inevitable; but by finding good band members you can put off the inevitable till (hopefully) you are rich and successful.
If you are in a band that has been together for a while and is looking for a new member, for whatever reason, try to get someone that one of you knows. This will help minimize any friction later on and also comforts everyone that the ‘new guy’ is vouched for. If you are putting together a completely new outfit, it would be wise to start with one person, get to know them and then when you are sure you want that person, take them and go hunt for the next one, and so on.
Although time consuming, this method will help you in the long run and help achieve a level of harmony within the band.