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

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didn't you say that that game took a year longer than you expected? i'm not it counts as missing only "a single deadline" if you missed it by one year :p
((and yes i'm not one to talk))
that said, i agree with you for the most part. a couple of additions though:
i think deadlines keep you on track even when they aren't met -- for instance, when i did immortal defense, the original deadline was to do it in 6 weeks (crazy, i know). it wound up taking 7 months, but if i hadn't set the deadline of 6 weeks i never would have done it even in 7 months. so in that sense "made to be broken" makes sense in that even if you break it, it still benefits someone more to have set a deadline and broke it than to never have set one at all
also i think that short-term deadlines are more effective than long-term ones -- doing X, Y, and Z by the end of a month is a lot more achievable than doing A through Z by the end of a year
so short-term deadlines like that at least keep the game moving, i can't imagine making games without any sort of deadline at all, just working on whatever part of the game seems fun at the time. there are developers who work like that, but i can't really work that way myself, it seems incredibly dangerous to work that way (and often when a game is abandoned or never finished usually they were worked on in that way)