"Ghostrunning" is the practice of deliberately allowing a Ghost to enter a level and convert normal gems into ultra-valuable diamonds. Because Ghosts kill the Spelunker in one hit and can freely pass through walls, ghostrunning is an extremely dangerous technique, but it can result in amazingly high scores.

The technique is threefold:

1. Don't pick up any gems until the Ghost enters the level;
2. When the Ghost appears, lead it over the gems;
3. Don't allow yourself to get cornered.

Because ghostrunning is practiced in order to maximize scores, ghostrunners will also want to visit the City of Gold. Sadly, the Hedjet - which is required in order to gain access to the Golden Door and thus the CoG - prevents the Ghost from appearing, so the vastly inflated scores will end after the Moai level.

To ghostrun, you need the following:

1. Bombs (to expose gems and dig to the exit)
2. Ropes (to get to gems and evade the Ghost)
3. The Udjat Eye and/or Spectacles (to discover gems which can then be bombed out)
4. High favor (so that you can obtain 99 bombs as early as possible)
5. Practice in manipulating the Ghost's path
6. Luck

The Tunnel Man's innate digging ability makes bombs less necessary for a successful ghostrun. Ropes, however, are just as necessary.

DeLGato232, the originator of the term "ghostrun," created a thread at the official Spelunky forums which illustrates the method. It can be read here:

http://mossmouth.com/forums/index.php?topic=118.0