It's never fun to get turned down for a date, but nor is it pleasant to be the person doing the rejecting. Uncomfortable date rejections can make your future interactions awkward, especially if it's someone you respect and see on a regular basis. Learn how to say "no" both nicely and firmly so that you can navigate these situations with ease and leave the asker feeling as confident as possible.
When to Use Excuses
Having a good excuse can take the whole date question off the table with no hurt feelings, but be careful: a bad excuse can create more problems later. If you've got a watertight reason that you aren't available to date, such as being in a relationship, married or of the wrong orientation, go ahead and say so off the bat. Unless he specifically asks, avoid any discussion of whether or not you'd be interested in dating if you were available; if it comes up again when you are available, you can let him know then whether you're interested, but for now, it's more appropriate and tactful to let the matter rest.
When declining a date offer, it may seem more tactful to be indirect than to directly say that you aren't interested, but understand that it's more hurtful in the long run to give him false hope. Make sure to use words that convey the meaning without ambiguity, such as "I'm afraid I'm not interested" or "I don't think dating is right for you and me." Apologize for the let-down and be as polite as possible, but don't let him talk you out of a firm resolve. Repeat yourself, if necessary, or say, "I'm sorry, but my answer is 'no.'"
Building Him Up
Pad your rejection with some kind words. Thank him for asking you and tell him that even though you're not interested, you're flattered that he asked. Depending on the relationship you have with him and what you think of him, consider giving him some reasons why you're flattered and why you consider him a good dating prospect and good man in spite of your lack of interest. For example, you might tell him that you enjoy his company and consider him attractive, but don't personally feel chemistry or attraction. Tell them that you're glad he asked and consider it an ego boost to be asked in general.
The Issue of Asking
If he seems to be hurt or embarrassed, show him some empathy and understanding for the uncomfortable position that a date rejection puts him in. This could be as simple as saying, "Hey, I know rejection sucks. Don't feel weird about it, okay?" Make an effort to be casual and comfortable around him in the near future, but give him space to recover from the rejection. Remember that disappointment in this situation will be inevitable for him.