See this:

http://extension.oregonstate.edu/gardening/how-build-simple-pond-native-frogs

Adults like cool surfaces like a sheet of metal set against the shady side of a building. They also tend to show up inside greenhouses.

The best way to introduce frogs to a container pond is to put some of their egg masses in it. Only works in the Spring, of course.

thanks all, I dont want to introduce unless the conditions are just right and I dont end up killing the little buggers. I'll just create the environment and see if they come along all on their own.

They are easy to attract in Portland. I have multiple small ponds full of them. Easiest is to sink a plastic whiskey barrel liner add some rocks and plants and wait for them to show up. You need it to establish some algae as that is what the tadpoles eat. I have given tadpoles to many people with a very high success rate. They return to the pond in Spring to spawn but disperse throughout the garden the rest of the year. They are loud, we love them but new neighbors might not but we've had them since the late 90's.

dug a hole, dug a trench around the inside edge, laid in liner and then stacked flat rocks in the edge and folded liner back towards pond and covered with another rock. This gives the froggies a place to hide and you never see liner around the ponds. This pond is about three feet by two feet by a foot and half deep. Has dwarf waterlily in it.