Great discussion topic. Here's my system in three easy steps:
1. If an email takes less than 1 minute to respond I just do it
2. If an email takes more than 1 minute to respond then I drop it in one of four folders: today, tomorrow, next week, or next month. I will then respond accordingly.
3. If an email pertains to a specific project that I've made time for in my calendar, I will drop it in a folder with the name of the project and respond at the appropriate time.