Things are going to pop up during the day no matter what productivity method you use. If you have 8 hours in the day and at the start of the day are listing 8 hours worth of things you want to get done, you're not taking into account the fact that new things will pop up. Could it be that you're setting unrealistic goals for each day?

If you're already working in a very efficient way and things are piling up, perhaps you're taking on too much. I used to think that GTD would allow me to take on and complete an *unlimited* amount of work. But of course no system can do that (although it has still let me become one of he most productive people I know). I have a list of questions to ask myself during the weekly review, and one of them is now "Are you taking on too much", which i find useful for when things are piling up for me.