actually my parents have done it. They were a model of what NOT to do.
It takes patience like you wouldn't believe. You need to be able to deal with a lot of different situations. Things like drug overdoses, developmental delays, sexual abuse etc.
Often you'll have kids with emotional and behavioural problems, so you need to be flexible, and understand that you may have to change rules and lifestyles to suit them. You need to make sure that yelling and violence is NOT a part of your lifestyle. I would suggest taking some courses in conflict resolution.
Another suggestion is to set rules, and make sure you stick by them. For example, if there is a rule that homework gets done before going out with friends, you need to make sure that the kids do their homework, or they have to suffer set consequences, such as no TV for the rest of the week.
You need to be prepared for nightmares and wetbeds, and understanding is a must. Expect frequent doctor's visits, and temper tantrums.
It's hard work, but in the end it's truly rewarding. I don't think anything feels better than a five year old hugging you and calling you 'mummy'.