I have an Overton which has started to get a bit hard to blow in the upper register. I have done my best to clean it but so far not much improvement. Any ideas? I feel like maybe it needs a good soak in something. 
okewhistle, what does it look like inside the airway? Any gunk in there could make a difference. You could soak the fipple end in soapy water but truthfully the most effective way is a mechanical scrubbing out with something abrasive enough to clean the gunk without being too hard to risk scratching up the airway. There are various products meant for other uses that would work well. Pipe cleaners that do not have the wire core but plastic cores would work.
A common household solution that I have used works real well on my hoovers (narrow windway) . Find a piece of straight plastic of medium softness and very thin thickness such as that clear plastic material that many products come in nowadays. You can cut several strips off of it which are slightly narrower than the windway. Then take a strip of lightweight cloth cut to the same width, dip it in warm soapy water. Then lay the cloth over the plastic strip and hold both together with your fingers. Then work the strip with the cloth in the airway until clean. No need to rinse this out as the soap film helps with miosture clogging anyway.
good luck… 
I have a tunable low D Overton, what do you have?
I routinely clean my Overton about every month. I take it apart and soak the entire whistle in hot soapy water for a few minutes. Then I use a strip of plastic that I cut from a credit card and carefully scrape out the windway. then I put it back together and hold it upside down under the tap with the water jet going straight down the tube while holding all the tone holes closed and forcing the water out through the windway.
Then I submerge the fipple end in the soapy water and let it dry without the rinse so the soapy water dries in the windway to help prevent clogging. Cheers, Cyril.
I have cheapie whistles, Waltons and Clarkes and I have recently discovered that I can clean them really well using bits from my husband’s gun kit. It contains a mild scrubbing rod I use to gently scrape down the inside with when needed (every few months). There is another long rod which holds a tiny cloth I use for wiping moisture away inside which I try to use after every time I play. This cleaning technique has revived my Walton that I thought was a gonner. So if you have a gun cleaning kit in your posession open it up and see what you can use!
Boatgirl, you are right about a gun cleaning kit being great for whistles. And you can find them pretty cheap too. I especially like the fact that you can add rod sections to wipe out even the largest whistle.
For people not so familiar with these kits, I have a couple of warnings.
First, the insides of guns tend to accumulate lead and other nasty chemicals. Using your kit for both guns and whistles might not be healthy.
Second, gun barrels are made of relatively hard material, so you can scrub away with stiff metal brushes and not harm them. No so with the soft aluminum - brass - plastic used in most whistles. I imagine wood would not respond well either.
You can find nylon brush ends, but they are not common. Brass brushes might be OK for occasional cleaning of brass or copper tubes, but I would not use them on soft aluminum. You get the idea.
You may find, as I do, that forcing cotton patches (made from old t-shirts) through your whistle tubes will keep them so clean that you don’t need to bother with the brushes.