ANSWER - There are a couple of areas in both categories now I stop to think about it. Magic that hasn't worked for me is usually magic that involves trying to make animals do what I want them to. I've noticed that people are a lot easier to manipulate than animals. Interestingly enough the more intelligent the person the more they respond to manipulation by magic. I have no idea why this is but my personal experiences bear it out. I've also had very little success with weight loss spells, probably because no spell will work without mundane as well as magical effort and I'm both too lazy and too greedy to cut down on the chips!
In terms of spells I wouldn't use, I have never cast any form of love magic for my own personal gain. I dislike the idea of creating an artificial relationship and I can think of no quicker way for anyone's confidence in themselves to plummet than the realisation that they are only loved because somebody has magically compelled a person to love them.
The other spell I would never use would be anything to bestow on me the ability to drive a car. Some people are just not safe behind a wheel and I can tell you categorically that I would be one of them. I wouldn't want to give myself a helping hand to pass a driving test only to find I am a danger to myself and others on the road. I'm happier on the train and besides, I answer a lot of the Ask a Witch a Questions on the train, I wouldn't get through half as many if I had to drive everywhere!