Nora, since you've got this issue in so many different places and you're concerned about your timing...take a look at this as an option:

http://www.leerburg.com/815.htm

The good thing about this is that the correction from the collar will be timed very well, more consistent than you as a human can be, and it won't matter if you're standing right there to correct. Can be used in the car, too, where giving a well-timed and proper correction can be difficult at best.

I know this collar has made a huge difference for some people. Give it a shot. Perhaps you could do this on your own, but it would take a lot longer, and I think it's not worth the effort when this is available to you and can be so quickly effective. Also, the dog will not associate the correction with you (in other words, barking like mad when you're not around to correct and making the neighbors nuts, etc.) Used properly, I also think it's humane and won't have the negative effects that an accidentally unfair correction given by you can have on your relationship with your dog. Not saying you've been unfair so far, but we're only human after all...mistakes can happen!

Just my (now awake ) two cents.