The real answer is to bill based on milestones instead of billing based on hours. Hourly billing in my experience is just a slippery slope to being way too cons

The real answer is to bill based on milestones instead of billing based on hours. Hourly billing in my experience is just a slippery slope to being way too conscious about time and creates a conflict of interest in that it is in your interest to take longer, but that isn't in the interest of your client. Billing off of milestones aligns interests because it is in both of your interests to complete milestones quickly. The only thing you have to watch out for is scope creep which can be easily addressed with contract clauses that address this.