Cacti need to get totally dry and then be completely soaked. Each time you water, which should only be like every 2-3 weeks if that, you should be putting a relatively huge amount of water in the pot. Totally saturate the soil when you do water--and all in one go, not carefully getting it moister as you might with other plants, just put it in the sink and dump in as much water as possible. The soil should be totally dry for awhile (even a week) before you water again. Forgetting about a cactus is better than watering too much.

And full sun, of course. =D