Someone once said that leading a group of people is like herding cats. In my experience, managing a band is like herding a thousand rabid and delirious cats. One has to worry about finances, promoting the band, showing shows, and managing the relationships between members. And that's all before you consider the extra responsibilities of musicianship when the manager is ALSO a contributing member of the band.
Financial management is something that most indie bands forget about, yet it's probably the single most important skill to have. Everything revolves around money; merchandise, payment from gigs, recording, etc. Having the right person to handle the money, even if it is not the main "band manager", is paramount to the band's long-term success. It would be a huge inconvenience to travel to an away gig and need to make an emergency purchase, only to find out the the band's bank account is overdrawn! Even if you are not great with numbers, but are still handling the money, there are countless software tools out there to help you keep track of purchases and income for the band.
Many bands simply promote shows as they come up, but an effective band manager will have a long-term and short-term plan of action for promotional purposes. This may mean scheduling interviews with radio stations or magazines, organizing giveaways for fans, or coming up with printed material and demo CDS to distribute ahead of shows in new markets to build up a fanbase. With the advent of the internet, Twitter, and Facebook, promotional tasks are significantly easier, but there are a LOT more bands promoting in the method. It's imperative that your promotional material be fresh and unique so that it will stand out from the crowd.
Booking shows can be an easy and rewarding tasks, or something that is done only because you absolutely must. The key to making it easy is effective contact management. Many contact management programs also will let you search geographically, making booking gigs really easy since you can "search nearby". Another thing to Incorporate is a simple booking agreement. Booking agreements protect you from sleazy establishments that may try to not pay or double-book performers (both of which have personally happened to me). It gives you a defined agreement that is enforceable by law if things go awry.
Bands will always have internal drama, whether it's someone's lack of self-practice or that thing that someone said to someone else. It's a manager's job to keep everyone happy enough to not quit, but it's also NOT his job to be a parent. Sometimes, you have to let some things play themselves out. However, if a situation appears to be affecting the well-being of the band as a whole, it's time to step in and take control, doing whatever you need to do to rectify the situation. Just take care and use caution not to implicitely take sides; simply act as a mediator. The reason is that you want to build people's trust in you to solve problems and not be a bad guy (even if you took their side).
A self-managed band can be a great thing if lead by the right band member. However, it's important to remember that your first priority is writing great music. When the management tasks start to interfere with that, it's time to start looking around for a real manager to begin the process of shopping for a major label deal!