What is a good way to remind someone to reply to your email?

It depends on what the relationship is like with the recipient. If you are fairly friendly you can be less formal.

Assuming you include the e-mail you are referring to in some way, you might say:

Have you had a chance to look at this yet?

This informally conveys the sense that you know how busy they are.

For a more formal version you would probably want to write it as a full e-mail:

John/Sir/Whatever you'd normally say

I was wondering if you have had a chance to look at the e-mail below yet? [Give a reason] Our customer enquired about it earlier today.

Many thanks,

Bob

Dear / Hi John (Use "Dear" or "Hi" depending on how professional your relationship is)

In regards to our recent correspondence as seen in the email trail below, please would you be so kind to let me have your feedback?

Your assistance herein is greatly appreciated. OR Id like to close finalise this matter and your feedback will be of great assistance.

Kind regards,

Ipsum Consult Team

I would not use a question mark; it implies impatience, frustration etc. I would say Just a quick reminder about your ___; thank you very much.