My chocolate mint is also growing very well and is very aromatic. The plant is in outside in full close proximity to my pepper plants. hmm..... anyone for hot chocolate mint. just kidding!!
I have let it get to almost wilting stage before watering, fertilized it with a dilute miracle grow every three to four weeks and leaves are small but prolific with great potency. I use the leaves for steeping with teas and other drinks "adult in nature" chocolate mint mojito is one of my favorites.

Bob I don't know if almost treating this mint like a pepper plant has retained it's flavor or if it is just plain luck but you can give it a try. By the way the climate is a little different down here too... so good luck.

Maybe it needs more sun?

I also have weak tasting mint, so I'm reviving this very old thread.
Is there anything I can do to improve the flavor?
I have a spearmint plant that I bought at a nursery last year, and have since planted it out in my yard on a hillside that I don't want to mow anymore. (yes, I am fully aware of how invasive mint is, that's why I planted it there) The mint gets full sun, and I water it on the same schedule as my vegetable garden. The soil it is planted in isn't the best quality though, and is mostly clay.
Any tips on how to make the flavor stronger would be greatly appreciated.

If you planted it last year, I wouldn't water it at all unless there are extreme periods of heat and/or no rain. It should be well established and able to survive without regular waterings from you. Let it get a little more stressed rather than coddled and water-ladened and see if the taste is stronger.

By the way, a lot of plants do just fine in heavy clay. It's what I have.

FataMorgana