Let's say you have a site about fishing and you need some fishing traffic. Go to the library and find a bunch of fishing magazines that 1) have a classified section and 2) are a few months old. Check out as many as they'll let you.
Now, go home and type in every domain you can find in the magazine ads, especially the classifieds. You will probably find a few that have dropped. Go to Namecheap or wherever and buy as many as you can afford and redirect them to your site.
But what good is this?
Well, if the former owner paid for a magazine ad, you can bet that there are more ads floating around out there, maybe lots of them. People tend to keep magazines so although you won't tons of traffic, you will get a steady trickle of highly targeted traffic perhaps for years to come and of course that is golden.
This also works for books. Oftentimes authors or publishers will set up a site to promote or support a book but then once the book runs its course, the domain will drop and you can buy it. Anytime you see a domain in a book, be sure to see if it is available. A surprising number of them are and I've found several good domains this way. Funny thing is, many of the books will still be selling well at Amazon but the domains are still abandoned. Go figure.
By the way, I found two domains tonight that have ads in national magazines. They have a combined readership of about 2 million and the domains were totally abandoned. There are plenty more where those came from.
This works for all niches, I don't care what you are selling, you will find lots of good dropped domains if you give it a try.
Good luck & have fun.