Deadlines are meant to NOT be broken. If you get into habit of missing them, they lose any value and it's becoming harder and harder to keep the team motivated.
Some tips that worked for me (missed only a single deadline during the last project):
- Typical mistake is to set impossible or overly hard deadlines. When you know you are going to fail, you don't even try.
- Have the first few milestones relatively short and easy. Use them to gauge how fast you can do stuff in reality. Then plan further ones according to that data.
- Make sure that the deadline was discussed with each team member and they said they can do their part. Even better - make them set their own milestones.
- Try using shorter milestones if you have problems. It's easier to plan ahead for a week than for a month.
- Have a clear goal for each deadline. Something rewarding. Like a new demo, or another level fully playable.
- The above also helps in determining which tasks are crucial and which can be dropped in the worst case scenario.
- Don't forget to include weekends and holidays in your plans.
- When you set a deadline, take the amount of time you think you can get it done and multiply it by two. It's to accommodate for unexpected issues, additional tasks that pop up, bugs, etc.
- General management tip: don't be an asshole about deadlines. Show people some trust. If your artist doesn't do anything for several days and then crunches overnight to get everything done, that's not your business as long as he's on time in the end. Most people hate to be nagged. If someone constantly misses their deadlines, just fire them.
- If you lead the team, try to never miss a deadline. Nothing demotivates more than a leader who seems to be slacking.
- Have a clear list of tasks that's available to everyone. Tick off things that are done daily. It's motivating and satisfying in that gamey way to see how things are progressing.
Hope it helps