What is a 'foolproof' way to balance any combustion reaction?

While balancing equations by 'observation' or 'trial and error' is possible for simple compounds, it helps to have a general method of balancing these equations, so that, regardless of how complicated an equation looks, you feel confident approaching it. Note that you can use fractions to balance equations.

1. (optional step) if the reagents are given to you in words, (e.g Write a balanced equation for the incomplete combustion of hexane, to produce carbon monoxide) you have to write it in stoichiometric (chemical equation) form,  initially ignoring coefficients (the ratios). This is where reading the question properly is key. If it's COMPLETE combustion the products are CO2 and water, if it's INCOMPLETE it's either CO, or C instead. 

2. Balance the carbon first, then the hydrogen and finally the oxygen. 

The reason you leave the oxygen until the end is that it's the only reagent that's made up of one type of atom, meaning you can put any number in front of it and you won't have to go back and balance the rest of the equation. You start with carbon because it's only present in one other species. This method is designed so that once you've written a number down, you never have to go back and fix it. So, regardless of how complicated the starting material is, the question doesn't take much time.