Hey Dude,
I had similar problems. I had a seriously bitter, sarcastic and witty sense of humour and for those that didn't know me, I sometimes came across as an arsehole. John Cleese might be funny in Fawlty Towers, but in the real world he would come across as a seriously rude prick!
Being a psychology student, I decided to take a psychological approach to the problem.
The way I broke this was working on my inner game. I realised I was incredibly cynical about the world due to a number of insecurities. When I realised it was a product of fear and it was holding me back, it became easier for me to control. It was mainly a habit. If you catch yourself out as your doing it or recognise it after you have done it. And stop, you will slowly get out of the habit. And don't stress, if you dont like your personality, it can be completeally re-built by breaking out of negative patterns and ways of thinking. Read up a little bit on neuro-plasticity if you don't beleive me
Also, would you care to write down some of the negs you used and in what context? Perhaps do the same with some of the jokes as well? Maybe then we can help pin-point the root of the problem more.
But just in general, don't be negative (sarcasm will be seen as negative) when around new people until they know you and realise your joking, then it might be acceptable, but only sometimes and around the right people. If your speaking to a happy up-beat girl, she probably won't be too impressed.
A book you might find helpful in getting into a more positive frame of mind is Authentic Happiness by Martin E.P Seligman. I found that seriously helpful in breaking my bad habits. It also has a number of tests you can do for free online as you read through the book to help you with your progress and to demonstrate the changes that are taking place.
Please note, this book isn't one of those positive psychology books written by one of those annoying up-beat manic american psychologists that con themselves into happiness. This book approaches it in a scientific and realistic approach. The guy who wrote it is considered the father of positive psychology and has always considered himself as a cynic.
Anyway hope that helps!
EDIT:
Also can I just say you always need to keep context in mind:
Some things aren't acceptable around new people (general negativity, bitter, cynical, sarcastic remarks).
Some things are not acceptable around most girls, especially those your trying to game. This includes things such as jokes that might be seen as crude/rude etc. You don't want to come across as a complete pervert. Ive seen guys make these sort of jokes a lot and it reinforces it for the girl. What might be acceptable at the pub with your mates isn't necassarily going to be for when your trying to game.
Like was mentioned above, don't over do the jokes or you become the clown and they don't take you seriously.