Writing from Twitter, Jocelyn asks:
The most likely problem that you are having is not so much one of the water in the ingredients as it is from the air itself. Some ingredients are hygroscopic, which means that they love humidity so much they'll pull the water from the air to bond with its molecules. Enough humidity, and you'll have clumps. This is why powdered milk is generally stored in individual packets, rather than in a larger container that you could scoop out.
One option is keep the milk powder separate. I'm presuming you're keeping your mix in an air-tight container, and you scoop out enough as you need then seal it back up each time. What happens in this case is that, each time you open it up, you let more humidity in. So you are probably, depending on your climate, okay for the first batch, but subsequent batches get clumpier and clumpier.
If this is the case, your first option is to store individual servings of your mix in their own containers, such as a smaller hard plastic container or perhaps a trustworthy zip-top bag. Presuming it's not too humid when you make the mix, that should take care of the clumping.
Option two is to store the milk separately. This is kind of a two-stage mix; not as convenient as a "scoop everything out at once and mix with liquids" option, but you have the milk in a secondary storage container as an individual serving, then add in the milk packet just before the liquids.
Option three is to dehydrate the storage container. You can do this with some corn starch, which is tremendously hygroscopic. Baking powder is generally a combination of baking soda, a powdered acid, and corn starch. The corn starch doesn't help with the lift at all, but it does keep the water away from the acid, which helps the baking powder to last longer. I'm not suggesting mixing the corn starch in with the mix, though, because it will mess with the consistency. While you could probably play with the recipe to make it work, it's really not worth it. Instead, what you want to do is to essentially make a tea bag filled with corn starch.
Get a decent paper towel and cut it into quarters. put a few tablespoons of corn starch in the center, then pull up the edges. Twist everything together, and tie with some butchers twine. If you leave yourself enough butcher's twine, you can just dangle the bag from the top of the container inside, and it should absorb a lot of the moisture before it gets to your powdered milk.
You may also want to consider changing your brand and/or type of powdered milk. If you are using fine powdered milk, you could use course, which doesn't clump as readily. Also, I'm told that Milkman brand dry milk was a very good powdered milk, but my understanding is that it hasn't been made for the past few years, so that might not be as helpful. You might give the King Arthur Flour Company's Baker's Special Dry Milk, as it's made for baking, though it's primarily made to work well with yeast. It's an experiment, but I get the feeling based on reading the product description that it's one that will work well.
I would be remiss if I didn't mention that I think that you are sacrificing quite a bit of flavor by using powdered milk instead of proper milk or buttermilk. Using real buttermilk would make for the tastiest pancakes, in my opinion, but I understand the allure of a very simple, very compact pancake mix.
Good luck, and please let me know how it turns out.