The common practice of making spoken announcements during Mass can be an important part of being a mission-oriented community, but it is most often abused.
Do Them at the Right Time
The General Instruction on the Roman Missal states that, "When the prayer after Communion is concluded, brief announcements to the people may be made, if they are needed." They should not occur before the prayer after communion, as is sometimes done. They should also be brief.
Don't Project Desperation
I know that many parishes are frustrated that their members don't do things they want them to. But pleading, guilt trips, whining, and nagging will not fix that. Actually, those approaches are counterproductive because they make the parish leadership look desperate. That communicates that people are not responding (perhaps for good reason). Saying "you should come" (because I said so) and "trust me" are not helpful. You need to give good reasons for responding that matter to the people in the pews.
Avoid the Details
Almost all parishes distribute printed bulletins at each Mass containing details of parish events and anything that would be part of spoken announcements. Details should be left for the printed reference (people won't remember them from the spoken announcements anyway).
Don't Abuse Them
Announcements disrupt the liturgical, prayerful flow of the Mass, especially if they are long. They also are very often experienced by the assembly as being disrespectful and annoying. Therefore, every word spoken in announcements had better be important.
The only valid reasons to include a topic in spoken announcements are to simply raise it to people's attention and to convince them that it is important. When the former is the reason, use minimal words. When the latter is the reason, add (briefly) why what you're announcing would help someone in their mission to live as an intentional disciple. If you find yourself reaching to make the case, what you're pitching probably isn't worthy of the announcements.
When you choose to include something in the announcements, you are communicating a priority. At some parishes, it would seem that bingo is the most important event happening there! You have to prioritize and be selective. If there is nothing important to announce, don't announce anything! The less you announce, the more effective the announcements will be.
The only announcements worthy of inclusion in the liturgy (with the possible exception of preventing cars from being towed) are those closely connected to the evangelizing goal. This particularly includes highlighting events that are well suited to which parishioners can invite seekers.
Get In Their Shoes
If you prepare announcements, you are most likely not an everyday lay person. What happens in the parish is largely your life, so you come from an assumption that everything happening in the parish is important. Try to get in the shoes of someone with a busy life, a secular job, parenting duties, and other responsibilities. Try actually talking to parishioners one-on-one, especially ones who are not very active. Ask them what would motivate them to come to an event.
Don't Try To Be Funny
Really, don't. This makes the announcements longer and usually competes with your efforts to sell what you're presenting. Leave the humor for the late night hosts.
Prepare Very Well
Consider putting as much forethought into announcements (at least per spoken minute) as homilies. Lack of preparation shows.
Announcements should be short, necessary, and generally of concern to all in the community. Longer and more particular information is better communicated in other ways, e.g. through the parish bulletin. Since the ambo is reserved for the proclamation of God's word, the announcements are preferably given elsewhere.
The Mystery of Faith
Federation of Diocesan Liturgy Commissions
The best announcements last no longer than 30 seconds each. That is the length of most television commercials and it is the length of most parishioners' attention spans for advertisements, no matter how worthy.
Modern Liturgy Answers the
101 Most-Asked Questions About Liturgy