I had the same problem, especially with Dave's Flexament and the Dave's Flexament Thinner. What I do now when I'm not planning to use the products for a week or more is to wrap black electrical tape around the neck of the glass bottle and up onto the lid. I get about 3 layers covering the joint between the glass and plastic lid sealing the lid - and this has stopped my evaporation problem completely - or at least it has stopped it to the point where I no longer notice that any has evaporated. I now do this with any "volatile" (prone to evaporation) fly tying products like loon hard and soft head, sparkle nail polishes etc. that I do not use on a very regular basis. Hope this works as well for you as it does for me.
