- Don't make up a lame excuse. Respond honestly and in a timely manner. It's OK to say that you've been really busy lately and you need just some down time.
- Thank your friend (or the party's host) for the invite. It's flattering that they thought to invite you.
- Schedule another time in the near future to catch up.
- Send a thoughtful note or a small gift if you were invited to celebrate a birthday, new baby, graduation, or the like. This shows that you're still thinking of them, even though you're not there in person.
- If you're skipping a party, follow up with your friend a few days afterward to see how it went. Who doesn't love bragging about the great party they threw?
Tip: If you've bailed on a particular friend a few times in a row, it might be time to suck it up and attend the party (even if you don't want to). Remember that you have to BE a good friend in order to have good friends.