Criteria to evaluate consulting opportunity
Balance is always tricky but basically the guidelines that I follow are:


If the consulting will help me learning something new, +1 
If the consulting will pay me a lot of money, +1 
If the consulting will improve my reputation, +1 
If the consulting will allow me access to something I want (new data), +1


On the other hand, 


If it takes a lot time, -1
If it offers little or no pay, -1
If it could damage my reputation, -5
If it would be a bit boring, -1


In the end, I just do the math.