There are several common mistakes happen at engagement parties that can be avoided if you deal with them in advance. Here are some commom advices:

Keep the party small

Engagement parties should be intimate gathering those who are closest to bride and groom, too many people at this event without scheduled activities can be disastrous.

Introduce the families in private

If the bride and groom parents haven't met before don't do the introduction in front of the audience. They can make that in private setting to bond without interfereing and comments from the others.

Don't overspend

You can save elaborate decoration, food and entertainment for the wedding and reception. The purpose of the engagement party is to intoduce the two families and let them know each other.

Keep it simple

The engagement party can be short and sweet since the purpose is to share your good news, show your ring, have nice time and there is no need for to provide any other entertainment.