This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
The worst connectors are push-down, or spring terminals. Screw
terminals with solid copper wire are much better. Gold-plated
binding posts and gold spade lugs are inexpensive by audiophile
standards and are extremely stable. Binding posts with spade
lugs can be tightened to get a very good mechanical joint, and
may offer the lowest electrical resistance of any connector.
Gold plated banana plugs and jacks are very good speaker
terminals. Good ones are more expensive than gold spade lugs,
however, they also provide a bigger area of contact, and are
more convenient when you must frequently reconfigure the system.
Banana plugs should be periodically monitored for corrosion and
loss of spring tension. Monster offers a banana-plug connector
with an expanding center pin that forms an even better
connection than common gold banana plugs. At approximately $25
per pair, the Monster banana plugs aren't a budget connector.
All else equal, connectors with gold surfaces are better than
connectors with any other surface. This is for two reasons.
First, gold is extremely inert, meaning that unless gold is
exposed to very harsh chemicals or harsh vapors, it will not
corrode or oxidize. It will remain a pure, low-resistance
conductor. Second, gold is quite soft, so that if a
gold-plated connector is squeezed between two metal
surfaces, it will deform slightly to fill scratches and
voids, giving a very broad, low-resistance contact area.
Corrosion of connectors is often a problem. Gold-plated
terminals and connectors somewhat avoid this problem;
problems with other connectors can be mitigated by
unplugging and replugging the connector on a regular basis,
cleaning the contact areas with a pencil eraser, or by
using a contact enhancer such as Cramolin or Tweek. When
you use a contact enhancer, be very sure to follow the
directions, and avoid spreading enhancer about your equipment.