A good way I've found to try and deal with impaction without worrying about accidentally harming the frog is to make sure the frog has the option to move away at any point. If you are pressing too hard or hurting something, your frog should just jump away. Trevor is exceptionally calm(lazy) though, so this might not work if you have a frog that normally tries to shy away from your hands or get away from you.
Another thing I've learned works is a sort of enema. I've had to do this a couple times, as Trevor almost never goes on his own. It is routinely two to three weeks between times (he is illegal to keep at my college apartment, so my parents are taking care of him till the summer. It's about weekly when I can monitor things) and I'm starting to believe he's got some sort of disorder or that his system was damaged by the really bad moss impaction he had when I got him from the petstore (it isn't his diet either; he eats dusted crickets and dubia roaches. Won't even glance at mice.) Anyway, when it gets really bad, I can take the spray bottle I usually use for misting, and put it at the cloaca (NOT inside, but you can gently pull the skin around the edges to make sure the water goes in) and spray one or two times on very low pressure. This is kind of my failsafe when soaking and massaging won't help. It's never not worked.
Again, this has the potential to hurt the frog, so make sure it can move away freely. They won't just sit there if something starts to bother them.
It seems like frogs that get constipated or have impaction issues tend to keep having them, so it's good to try and find something that works since a vet visit every time just isn't practical, especially if you keep a lot of other animals.
Good luck, and keep us updated!