Charging by the hour vs fixed prices

Ever had a fixed price project go over so you're no longer making money on it?
Or, if you charge by the hour, been overwhelmed by change requests?

There are two main variables on every project - scope and price.

You can choose variable pricing - where the client pays you by the hour, by the day, by week or by the month.  
Or you can choose fixed pricing - where you what the client pays is agreed in advance.  

You can choose fixed scope - where you define exactly what the client is going to receive in advance.  
Or you can choose variable scope - where you receive change requests whilst the project is in progress.

Fixed scope, variable pricing is bad for your client.  
Variable scope, fixed pricing is bad for you.  

So either choose variable scope, variable pricing.  
Or fixed scope, fixed pricing.  

But make sure you choose.