Its been my experience that "hunting" on the brink of a storm, then catching one out in the storm when it comes is always worth a low ball price request, but you typically get suckered into the whole "give me a ride to my friends house" bit.
Now I dont need to tell the veterans what adventures lie in wait on that trip to a friends, but In order to get out of the rain, a typical .60-1.0 encounter can be had for .20 to .40.. If shes a low rent girl already, then its possible to get a quick stick or sucked on the way to friends house for as little as .10, but you'll have to work for it.. Ive given plenty of rides, left plenty at the first gas station on the way, just play it by ear, but siezing an opportunity pays off most of the time. Its the amount of effort that goes into finding it (and luck is still the main player).
As for heading out after it starts raining? well even sw's typically know when to get out of the rain. Take that opportunity to cruise through motels looking for open doors and girls smoking on the porch. Probably wont get a deal though because they know they are out when others arent and they have a room right there. But I dont mind paying for convenience either