Dry firing definitely has a use.  I find that slinging, at least in the early stages, is an art particularly susceptible to decay.  All primitive skills are living skills which fade quickly with disuse, but slinging especially so for me.

Dry firing is a great exerciese when you can't sling ammo, just to keep the motion fresh in your head.
 I normalay sling at least an hour a day, everyday, and I'll go through sling withdrawl where it's all I can think about untill I break down and start dry firing if I go more than a couple days without slinging.
 
You can even dry fire in a hotel room if you need to, but just like anything else it is a perishable skill so keep practicing.

I dry fire everyday as part of a warm up and on those days that I can not go outside I have been using a pingpong  to gauge my initial trajectory in the house. One major problem practicing in the house is limited overhead room. Even if I am not slinging with full force, at least I get to work on timing and form.

Quote from rockslingerboy2 on Apr 9^th, 2012, 8:12pm:

 
You bring up a good point. I have noticed something similar. I would guess most of us have actually. I've heard several explanations as to why this occurs, though as of yet I don't know which to believe.  

It's probably some sort of neural fatigue that occurs when you sling too much, the neural connections in your brain need time to recover after each slinging session and if you sling again too soon, you'll notice a deterioration in performance. Leave it too long and the new connections that had formed may have had time to break up again. That's how I think if it anyway, a bit like the same way muscle tissue responds to strength training - the training stimulates growth, but if you train again too soon you overtrain and your strength deteriorates, if you wait too long your muscles have time to atrophy back to their original size again, it's all about getting the timing right. I know that with strength training one technique for gaining strength is to blitz your muscles for a week, just keep training hard every day for hours even though you're getting way weaker and really suffering mentally and physically, and then take a whole week or two off. Some people report coming back remarkably stronger after the 2 week rest period. I imagine a similar effect might be had on slinging performance.

Quote from Thearos on Apr 7^th, 2012, 1:57pm:

 
That is a capital crime here.  Actually that topic might warrant it's own thread.
 
I never could do dry-runs without a sling.  I have trouble imagining where it's going to go.

I take 4 to 5 months of every year, it is called winter and I enjoy doing other things during that season than freezing my fingers with snow balls.
 
But basically when I start again it takes at most an hour to get the feeling and timing back for me.  What I do notice is I have to do gentle throws for a week at least or I will pull something in the arm.  But then again I am getting old and have to warm up for everything.