if buy from dealer, ask them to do for you pay a bit more money (a little)
if private sell, first you have to agree with the buyer that you keep the number plate and normally you pay for the related cost.
then when you go to do the tranfter at TD, just tell them 'keep old plate' and transfer the title of the vechicle.
The will then transfer the title of you car and issue a new number. you need to then go to number plate shop change the plates for the new buyer.
TD will give you a piece of paper with you old number plate on it, you can use this to register you new car with teh old number (need to do within one year or it will expire)