For the average person, NVRAM on most games is not a good solution. It requires the removal of the stock RAM chip (which is soldered into the board), a chip socket be installed, and then the NVRAM installed. These means you need to have a decent skill set and decent desoldering tools. Most people just don't have that level of skills. Also the NVram is expensive!
On non-WPC games, my weapon of choice is the CR2032 coin battery. I use this on nearly everything from Gottlieb system1, system80, system3 to Bally -17/-35/6803 to Williams system3/4/6/7/8/9/11, and all models of Dataeast/Stern games (except SAM, which already has a coin battery system installed.)
The price of CR2032 batteries is about the same as three AA batteries, roughly 65 cents. The battery holder is another 65 cents. (Obviously not Radio Shack prices, but from places on the internet.) So the whole system is under $2. The CR2032 system is small and tight, does not look out of place or un-natural, and mounts directly to the CPU board. No wires hanging off.
Since the CR2032 system is not rechargeable, on games that originally had charging systems, you'll need to use a blocking diode (Gottlieb sys1, sys80, Bally -17/-35/6803). I use a 1n4001 diode, which costs less than 10 cents.
On WPC games I use a remote four battery AA holder (about 69 cents.) In the fourth bay I put a 1n4001 diode, which lowers the overall voltage of the three installed batteries. This gives you a "head up" that the batteries are going dead, with "Factory Settings" game errors. This is important because batteries usually die first, then leak, usually not the other way around. So having a heads up often lets you know when batteries are going low and you can change them before they leak.
Another good solution for WPC games is the remote battery holder that Rob B. sells, as seen in a picture above. That's a good no solder system. My only complaint is it's price (which is reasonable for a couple or few games, but in my situation, owning about 40+ WPC machines, it would be cost prohibitive.) But Rob's product is excellent and a great battery solution for any WPC game, and is economically (as long as you don't have to buy 40 of them!)