I am on my second rescue, the first was a yellow lab and the second a shepherd. Max, the lab, was about 5 when we got him so he was more mature, very gentle and loving. A stern look was all that was needed with him and he did not get even that very often. I was devastated when we had him put to sleep at almost 14. When we decided to get another dog, I knew I would take care of it, but thought that I would never love another dog the way I did Max. We decided to get a younger dog the second time around. Newlie was described as 3 or 4 when we got him but we found out later he was maybe 1 1/2 or two. Sweet natured, good with people, but full of beans, stubborn and rambunctious, I had to learn to be more firm with him. It will kill me to lose Newlie just as much as it did with Max.
My advice:
Be honest with yourself, your lifestyle, what you want and your limitations. Don't just look at appearance, ask questions and try to find a dog that will work for you.
Be ready for some surprises, things are not always what they seem. But the same thing can be said when people get a puppy from a breeder. You have to be willing and able to adapt.
Building trust and a relationship takes time, there is no substitute for that.
Accept them for who they are, not who you want them to be. I'm talking personality here, not behavior. If a dog is high drive, you are not going to magically change them into a couch potato. If they are not demonstrative, you are probably not going to change them into a cuddle-bug.
Love them while you have them because with any dog, their time on earth is much too short.