The fear to refuse to someone reciprocity can force you to make concessions contrary to own desires. If you do not tell "no" in time, risk to become the puppet in hands of the one who is unpleasant or uninteresting to you.
1. It is always very difficult to speak "yes", especially, if business concerns private life. Let's say you should choose one of several admirers. Anyway, to someone from them you will refuse. Unfortunately, it is unlikely the one to whom refused will be able to make light of a situation, however, in your forces to hold this conversation adequately. Yours "no" has to be firm and unshakable therefore at first pronounce this word to the reflection in a mirror, and then to the real interlocutor.
2. Surely reason the refusal. Do not pour a long and incoherent mere verbiage, think over the laconic, but at the same time capacious answer better. You speak specifically, without streamline phrases in the spirit of "you and perfectly understand everything" or "we already once spoke about it", "in my opinion, all also is so clear", etc. Respect the one to whom you refuse as you can easily fall into its state.
3. You speak honestly. Slyness, cheap acting and offers with double sense in this conversation are absolutely inappropriate. They will obviously not lead to a constructive conversation and are capable to aggravate a situation only. Did you have another? And tell about it. Do you want to have a rest from the serious relations? Be sincere. Do not lie that you wish "to be alone some time (one)", and you will go to a party with the new satellite in the evening (-tsy).
4. To sweeten bitterness of refusal, begin it with a compliment or just pleasant words. In this case everything depends on a situation context. For example, tell too persistent stranger "You are so self-assured that my refusal should not disappoint you strongly". And a conversation with the former beloved, feelings to which cooled down long ago, begin with the words of gratitude for time which is earlier spent together. In other possible cases it is possible to tell approximately following: "You are surprisingly sensitive and gentle person, but, I am afraid, to us with you not on the way".