Making a decision before you know all your options is not easy or wise. This can happen when an employer gives you a job offer, but you still have other interviews lined up for even better jobs. You might be tempted to tell the employer you need more time to consider the offer -- but that does not mean that you should tell him the real reason you need the additional time.
To Tell ... or Not
Evaluate whether it is necessary for you to tell an employer about the other interviews you have lined up. You should only divulge this information if it works to your advantage or you have to explain why you are not ready to accept an offer. If you know the employer wants to hire you, letting them know you have other interviews could mean you will receive an offer sooner. However, in other cases, the interviewer might lose interest if they get the idea that you will accept any offer you get.
Ask For More Time
If you have a job offer, but want to hold out for a better one, you must find a way to manipulate the timing using effective negotiation. Express your strong interest in this position, but explain that you are still interviewing and need more time to make a decision. Make sure the employer doesn't get the idea she is not your first choice. Negotiate for the longest amount of time the employer will agree to. Ultimately, it is the employer's decision, and you must abide by it.
Contact the Other Employer
Contact the employer you are waiting to interview with and let him know that you have received a job offer elsewhere, but really want to work for him. Ask for the interview timeline so you know just how much time you need to buy from the other employer. If needed, ask if the interview can be moved up.
Accepting an Offer
Once you accept a job offer, you should be committed to taking the job. Interviewing elsewhere at this point is a bad idea and is considered unprofessional. If you want to do other interviews, delay accepting the job offer until you can make a commitment. Additionally, once you accept the job offer, it is impolite and unprofessional to change your mind and reject the offer later -- were you to get another job offer.