Unless you are following a specific plan, 20 lbs in a month is really hard to do. And while I agree with the others that if not done right it
can be unhealthy, if you do it right and in a healthy way, it is obtainable. While most medical experts do recommend a 1-2 lbs loss per week there is also a lot of medical evidence out there that says it is safe to lose between 2-5 lbs a week. I guess it's just something where you have to decide for yourself who you want to believe.
I'm on Medifast and on this plan the average weight loss is between 2-5 lbs a week and is totally safe. I don't have a whole lot to lose, though, so I've averaged about 2 lbs a weeks so far. (You can see my signature, it's slowed from the first week, but the higher number sure does
sound better! Gotta keep motivated!
That and I also attribute the slow down to TOM.)
Since Medifast is a pretty big commitment, and if you don't want spend the money on it, I would try Atkins or South Beach or another low-carb diet as those tend to be the safest
fast ways to lose weight. Or as wendymeows suggested, Weight Watchers at home has a good reputation also and I've heard is cheaper than going to meetings.
Good luck and let us know how it goes!