When do you buy the domain?

Tricky! I'd say after the product has started to take shape, I went through 3 domains before settling on one.

I tend to do the same mistake of buying the domain as soon as the idea swings by, i start my search for a good name and baam its bought :) thats why i have around 30 domains lurking around... haha

Every year around 10 of my domains expire and I add 10 more. On average I have 30-40 domains.

As soon as MVP is alive, working and ready

Right after I decide that I'm going to work on it. I don't buy a domain name because I have a "great idea." If I feel like I'm commited to it, then yes, I'll register a domain name.

Make sure you work on your idea tirelessly

Whenever there is a solid convention to proceed with the idea before the actual coding starts.

Right at the start. How $$ disappointed are you when someone registers a domain later for some other reason?

@aaronoleary on the positive side, maybe one day, your unused domain could give you some good return on investment ;) I would buy a domain once I know I will proceed with the idea.

I only have one domain so far, but I waited fairly late before committing to it.