Here are some things that you should keep in mind while declining a business invitation.
Respond on time
It is advisable to decline an invitation in time, well in advance before the event. It is bad practice to keep your host waiting endlessly for a response from you. Ideally, you should decline the invitation as soon as you come to know that you cannot attend the event. Responding within a couple of days of receiving the invitation is good etiquette.
Declining an invitation on time gives the host a chance to get through with the final headcount on time and make arrangements accordingly. He/she also gets the chance to offer your place at the event to someone else, in that case.
For couple invitations
When you get invited for a business event as a couple, with your spouse, and one of you cannot attend, it becomes necessary for the other person to decline the invitation as well. This is particularly so in case of social-cum-business events.
Responding to such invitations on time gives the host a chance to think over the situation. He/she can then offer your place at the event to some other couple or request the single person to attend the event alone.
Consider the mode of declining
When you decide to decline a business invitation, ensure that you do so according to the type of invitation. If the invitation includes an RSVP card, fill out the card and send it back to the host. If there is no such card attached to the invitation, you might choose to decline through any means convenient to you, such as over the telephone, through a personal letter or e-mail.
If the invitation is from someone who is especially close to you, it is a good practice to decline through letter or e-mail first, and later follow up with a telephone call expressing your regrets.
It is good etiquette to show regret while declining a business invitation. Instead of just stating that you will not be attending the event, state in your communication with the host that you are regretful for being unable to attend. If possible, include a line about the reason why you will not be able to be a part of the event.