I seriously would consider investing in coco-fibre. In all my years of herp keeping, I have always given my frogs coco-husk bedding. It is the best for keepign humidity up and it is the least dangerous substrate for your frog. Dog Shrink is right pine and cedar shavings can give off aromatic hydrocarbons - Phenols - which can be harmful to reptiles and even some rodents kept on this substrate - never mind frogs which such delicate skin - I mean, these guys breathe and drink through their skin, heaven knows what they'll take in accidently.

In regards to impaction - coco-husk is also the safest as if taken in by these ferocious eaters, it will just pass through. A problem with sphagnum moss is that when eaten by White's it is very hard to pass through, and can put your frog in a lot of pain. Bark isn't as big of a problem as moss can be, as it is rarely accidently eaten as it is so big, these frogs will generally notice when they've accidently got a bit of bark and swipe it away with their 'hand'. Another problem that can arise with too much sphagnum moss is that it can retain your frogs toxicity which is no problem in small doses, but once built up within the moss, can cause some problems to your frogs health. Also, I would not recommend putting any sand in your substrate! That will surely only irritate your frogs skin and just isn't a good substrate for a tree frog.

With regards to other solutions to raise humidity - I'd imagine with the exo terra terrarium you have described - your bulb is resting on the metal mesh at the top of the tank? I would recommend finding a slightly weaker bulb, as it could be contributing to the drying out of your substrate.

Finally, another way of raising humidity is placing something on top of the mesh at the top for a short while - always raises humidity quickly.