If you decide you don't want to date someone (for whatever reason), please do not offer the crappy excuses, "I don't want to date anyone" or "I don't want to be in a relationship right now." Especially if you are going to turn around and start dating someone else and not make a secret of it.
This does two things. It hurts the feelings of whomever you dumped in the first place. Secondly, it just pisses them off because:
A. You have just demonstrated that you are a lying douche bag.
B. You have insulted the intelligence of the person you dumped. Which only illustrates that you are not only a douchebag, but a dumbass besides.
A lying, douche nozzle of a dumbass. While you may have done the dumpee a favor and demonstrated that you are really not worth pigeon shit, you have still wasted the time of someone who could have been the most awesome person you will ever know. And while the dumpee may not spend any time crying over you, they will hope your tiny, unremarkable penis catches a yet unidentifiable STD and it drains green sewage, turns black and falls off.
In conclusion, just be honest. Honesty, while painful at times, is always the best policy.
Until you jerks can learn this lesson...you can kiss the fattest part of my ass. And the asses of the other girls you've done this to.
Have a nice day!!