Nobody likes to be stood up, especially at short notice. To avoid seeming rude, you must cancel plans with your friends in the right way. Regardless of whether, deep down, you really want to spend time with your friend, always show respect for her feelings. Striking the right tone when it comes to cancelling plans is especially important when you are still getting acquainted. You don't want to seem fickle.
Cancel plans as soon as you realise you are unable to attend. Don't sit on the information and allow your friend to get excited, only to dash his expectations at the last second. Regardless of your reason for cancelling -- a conflicting commitment, personal emergency or change of heart regarding the social event -- never delay your cancellation.
Contact your friend directly, if possible. Cancelling plans in person is best but not always practical when you both are busy. Call your friend and attempt to speak one on one. Messages left on voice mail or with assistants are impersonal and give the wrong impression. Text messages suffice in some situations if politely worded. Don't cancel plans with new friends by SMS.
Explain why you are cancelling your meeting. You don't have to go into detail, just tell your friend you have conflicting plans or explain why you had to give another engagement priority. Be honest, don't lie or kill off a distant, fictional relative for an excuse; you never know if your friend will catch you out later.
Apologise for the cancellation and make plans to get together another time if you really do want a meeting. Do you utmost to make the rescheduled date stick; it starts to look bad if you constantly cancel plans. If necessary, cancel with someone else to make the rescheduled date work.