Makeup is one of those things that always walks a fine line...'Did I do too much? Or not enough?' Obviously, with dewiness this is especially important because on one end of the spectrum your skin looks heathy and glowy, and on the other you look like a greasy mess. So how do you prevent yourself from dropping into that oily-looking territory? Pay attention to where you apply your products.
Dewy skin essentially boils down to creating a moist finish, which is especially gorgeous on the parts of the face that don't look oily when they have shine. Basically, anywhere outside of the t-zone is safe. If you are using a very dewy foundation, all you have to do is take away excess shine throughout the center of the face: the middle of the forehead, alongside the nose, and the chin. Use either a blotting paper (if the shine is only a tad too much) in just these areas, or use some powder if you really need to take it down a notch. If powder ends up looking too flat, don't forget you could use a bit of anti-shine gel and it'll do the trick also.
Another easy way to achieve a dewy glow without going overboard is to use a dewy setting spray (one of my favs is from Ciate). Try holding the spray about a foot and a half from the front of your face and only do one or two spritzes. Then, if you want a touch more dewiness, hold the spray a foot away from the side of your face and spritz again (on both sides). That way you won't build up too much glow through the center of your face.
Keep in mind that dewiness may increase throughout the day. Whether you get a little hot 'n sweaty, or your skin gets oilier as the day goes on, you may need to monitor yourself for a quick touchup to take away the excess.