A first rule of thumb, is to avoid any ghost towns that are easy access, easy research, etc... Like the kind of ghost towns you'd find in a Coffee table Sunset Homes and Gardens or Triple AAA maps type of tourist traps. A buddy and I traveled the desert southwest years ago (CA, AZ, NM) md'ing all sorts of places that struck our fancy. One of the resources we gathered up before we left on our trip, was a few glossy books with photogenic places like these. The first thing we learned on our trip was to AVOID these obvious spots. Anything that inviting, whether public or private, has been pounded to death, or had their doors knocked on ad-nauseum. Instead, we tended to research out little known spots that WEREN'T in readily avaiable sources (short lived forts that have no trace of their existence left, individual adobe sites in hard to reach places, etc...)
Another thing to keep in mind is that existing cities are often just as old, and just as colorful, as "ghost towns". The only difference is, the "ghost town" froze or died in it's growth, and modern cities still kept on. So if you stumble onto old town demolition sites in existing old towns, the goodies can be every bit as old and exciting as ghost towns. Of course, old-town-urban demo's are hard to come across and hit at *exactly* the right time. They might scrape too deep, or not deep enough, they might add fill-dirt before you get there, etc.... On our trip through the desert southwest, we ended up getting as many or more old coins from urban vacant lots, and demo. sites, than we did knocking ourselves silly looking into "ghost towns".
There was a time, back in the 1960s and into some of the 1970s, that "obvious" ghost towns were ripe. But nowadays, unless you live in a corner of the USA where there simply aren't any hard-core md'rs, no good site goes un-checked. You've either got to be an ace-hunter to get more from there, or find sites that other's haven't accessed yet. You can also try researching for trading posts, stage stops, etc.... rather than whole "towns".