In your childhood you've probably been told that work comes before play. This makes perfect sense - you want to teach your mind into thinking that it has to earn the pleasurable activities. It's also practical to do the tedious stuff when you've got more willpower because you'll need none of it to do the fun spare time activities after the work. (Interestingly enough, if you do the fun stuff first, all of the willpower will have vanished like magic if you try to do the work after it. How about that.)
So when I first stumbled upon the concept of advance rewarding, I was sceptical to say the least. What's stopping you from thinking, after having the reward without working for it first, "nah, I deserved a break, there's no need to earn that now... I'll work for the next reward", and then never will? What's stopping the reward-without-work mentality from becoming a habit?
Turns out, everything is. If you have a progressive mind - and you probably do, after all you're reading an article on a site that has the word 'progress' in its name - chances are you'll feel so much self-hatred for reversing the work-reward sequence that you'll end up making amends for it my working multiple times the amount you would if you'd done the work first, therefore 1) punishing your mind with massive work for having the reward first so that it'll learn to do the work first in the future, and 2) getting a lot more work done than you normally would.
It's a cute little hack if you know how it works. The most important thing to remember is that you mustn't let this become your default modus operandi. You might think using this hack all the time would increase the work you get done to its optimum, but as soon as you start doing this regularly it's effect starts to decline. The amount of work you consider adequate payment for the premature reward will decrease, sometimes exponentially, every time you do this.
Therefore I recommend you do this maximum twice a year.
You must have a more sustainable work-reward system at place in order for this to function, the most sustainable system in my opinion being first work, then play, and preferrably as polarized as possible - one month of hard work earns one night out, or two consecutive months of hard work earn two consecutive nights out, etcetera. (Or one month's strict dieting earns one pint of ice cream. Etcetera.)
This way, if something happens - you either screw up unintentionally or mindfully decide to bend the rules - you know it's not the end of the world. You just have to pay the full price, with interest.