These days the main method is to use an optical drive emulator like Daemon-Tools, although in the last few years there has been a bit of a game of cat and mouse between games blacklisting virtual drives and tools that allow you to 'hide' them (or disguise their virtual nature).

The other alternative is just making a really accurate duplicate of the original disc, by ripping and burning it with a utility like Alcohol 120%, which can make perceptibly identical copies of the game media.