Succulents are well-adapted to the conditions of the biome that they generally come from:  Deserts.

Light

Lighting ranges from direct sun, bright light, filtered-bright light, to medium light.

The succulent's size, leaf/stem color, and texture are good metrics for suitable light levels.  Succulents may be grown indoors and outdoors, though the majority have a predilection toward the outdoors.  A reliable guideline to go by is that green leafed/stemmed succulents can tolerate less light than tinted (e.g. black or red colored succulents) / textured (e.g. succulents with sheaths or hairy coverings) varieties.

Lighting Indicators

Insufficient Lighting

  • Etiolation - Some succulent varieties, especially echeverias, are known to "etiolate"/elongate as if to "reach" for a better light source when there is insufficient light.
  • Lightening - Plant leaves will discolor to lighter shades of green and eventually turn yellow (then brown ...then fall off) if left without sufficient exposure to light wavelengths.

Excessive Lighting

  • Burns & Callousing - Many like cacti and echeverias result in a woody callous where sunburned.  Thinner plant tissue will be more susceptible to burning so exercise caution as to where you place your plant.
  • Darkening - Varieties such as Haworthias and "Flapjacks" can deeply discolor to a reddish tinge to show signs of sun stress.  Sun stress can be ok if the plant doesn't begin to dry out.  In fact, sun stress gives many succulents their character.

Soil

Many succulents grow in rocky/gritty mediums with little soil organic matter.

A balanced soil mix that considers aeration, moisture/nutrient retention, and compaction prevention is advised.  Ensure that the pot/vessel has excellent drainage to prevent water from collecting at the base of the pot, which would thus result in rot.

Water

Water succulents thoroughly (until water runs out of the drainage hole of the vessel) when the soil medium dries out.  Do not water again until dry.

Most succulents thrive, believe it or not, when left dry for prolonged periods of up to two weeks or so.  The succulent's soil/medium, current weather conditions, and plant warning signs may, though unlikely, override the general 'wait until dry' re-watering rule.  It's best to water succulents as necessary as opposed to a set schedule as each individual plant's needs will vary.
NOTE: Ensure that the plant does NOT reach "bone dry" status, however, as this compacts the plant's root system and consequently compromises the plants' ability to absorb water.

Watering Indicators

Overwatering

  • Stems/Leaves are tender or excessively plump to the point of ease in rupturing plant tissue.
  • Stem/Leaves start to discolor dark brown/black and plant base is flimsy.  Leaves toward new growth fall off with a slight touch.  This is a sign of rot and usually signifies a dead plant.

Underwatering

  • Leaves toward the base of the succulent fall off more easily than usual.
  • Stems/Leaves are shriveling rapidly.  This is especially alarming if shriveling occurs near the growth point at the plant's center or top.  However, this can be rectified with thorough watering.