Cleaning out your email inbox is wasted work if it gets cluttered again in a day or two. It's time to hone those skills you used to achieve inbox zero in order to maintain a clutter-free email. I can think of nothing more annoying in terms of emails than having finally achieved peace only to look up a day, week or maybe a month later to see your inbox once again cluttered.

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Only look at email at dedicated times.

Having dedicated time slots for handling email is not just good for maintaining inbox zero, but great for your productivity as well. If you let emails trickle in all day long, you constantly get interrupted and you may look at a message without dealing with it.
Set up as many time slots as you feel necessary to deal with email. You know best how much mail you receive, you might only have to look at your email once a day or maybe you need to look at it for 5 min every hour. Try different systems for a week at a time to allow them enough time to see if they work for you.
I personally look at my mail most days around noon and once in the afternoon, unless I'm expecting an important message.

Keep responses short

Often you don't have to write a novel to respond to emails. Remember that your time is precious and so is that of the person you are responding to. Writing a clear and succinct response is faster for you (once you get in the habit) and will be faster to read and react to on the receiving end as well.
I've tried an experiment to see if I could answer all emails for a month in no more than three sentences. For most emails, it was completely doable, and for the few that needed more to make sense, it still reduced them significantly.

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Delete, delete, delete

I bet you got really friendly with that delete button of yours while achieving inbox zero, keep that friendship ever evolving by deleting as much as possible. If a newsletter or something similar comes to your inbox and it no longer provides value to you, then don't just delete it, unsubscribe from it. Most such emails will have a link at the very bottom that allows you to easily unsubscribe.

Archive right away

As soon as you've dealt with an email, you should get it out of your inbox, either by deleting it or by archiving it. Remember to apply filters and labels as well as creating new folders as the need arise.

Only keep emails in your inbox that requires action.

There is no real consensus, as far as I'm aware, amongst those who practise inbox zero as to what you should do with the emails that require more action that you can take as you are working through your emails. Some say to file these emails in a To-do folder, others say to transfer the task to your to-do list and then archive the email in the place it belongs.
My advice is slightly different. I quickly realised that I forgot to look at the To-do folder all the time and that transferring email tasks to my To-do list didn't work for me either. So I loosened the reigns a bit on the absolute of inbox zero and keep emails in my inbox as long as they require action. This forces me to see the emails every time I open my inbox and since I really am a raving fan of having no emails there, it makes me deal with the emails a lot faster.

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The key is to find a system that works for you, including how to file your emails and when to deal with them, and then being consistent. As long as you keep on top of things you'll never get back to that mountain of emails you started with and should you for some reason find yourself back in that hole, you already know how to climb back up and how great it will feel.

Now, I'm curious, what's your worst pet peeve when it comes to email?