Most of the important things were already mentioned here.
I'll just add

• Have everything prepared before the launch and have a detailed roadmap
• Don't neglect PR, marketing, business, tech, legal. Have a great (experienced) team.
• Have a perfect whitepaper but know that "nobody" will read it. Have a medium blog.
• Don't be just online. Go to meetups, conferences, talk to investors and techies. Form a strong community.
• Don't scam!

Also I find this article really helpful https://www.cryptocoinsnews.com/9-mistakes-we-made-during-our-ico-and-how-to-overcome-them/