There are a lot of reasons why you cannot go.
It may be that you had a really tiring day at work and just want to go home and collapse in bed.
It could be that you find the guy a real creep talking to you with his eyes on your chest.
It could be that you are dealing with a personal problem at home and not in the mood for a romantic dinner date.
Whatever your reasons are, you just do not know how to refuse the date without hurting his feelings (jerk or not).
Be nice about it Just because you are in a bad mood does not mean that you have to be mean about it.
Try to let the guy off as nicely as you can.
Explain to him the situation and why it warrants your refusal to his offer of a date.
Anyone who is a true gentleman will understand why you need to take a rain check.
Keep the conversation friendly and assure him that if another opportunity comes up that is favorable and convenient, you will be willing to go.
Unless you really do not want to go on a date with him.
Be sincere about it Especially when he is a perfectly nice guy and in other circumstances you would say yes.
Do not act nonchalant about the whole thing because you might come off as arrogant.
A bad attitude or even an insincere attitude may even prompt rumors in the office against you.
Always say thank you first for the invitation and follow it up with a gentle explanation of why you cannot go on that particular day.
Be private about it Do not give your answer in front of other people.
Avoid making a scene.
If you need to, pull the guy to a private place where only you can explain to him your predicament.
Refusing him flat in front of his friends or your friends can brew resentment and humiliation on his part.
There is no need to unnecessarily embarrass the person.
Be discreet about it Even if the guy is an unsavory character, do not circulate rumors yourself.
Keep the whole thing discreet.
All guys, who experience a refusal, will get a notch lower on their ego post.
Being discreet about a refusal will earn you the respect of the guy and even friendship.
Be honest about it If you have no other reason aside from the fact that you really do not like the guy, then say so.
Asking the person for a rain check, instead of refusing him outright will give him (if he is especially dense) the idea that he still can ask you out later.
This will create even bigger problems for you later on.
Better to burst his bubble now than have to burst in anger and frustration later on.
Of course, the above rules should also be used in tandem with this rule.