The ideal soil in a water-conserving landscape does two things simultaneously: it drains quickly and stores water at the same time. This is achieved by increasing the amount of organic material in your soil and keeping it well aerated. Compost is the ideal organic additive, unless your xeriscape contains many succulents and cacti. These species prefer lean soil.
It may be worthwhile to have your soil tested at a garden center or by using a home test kit. Most Western soils tend to be alkaline (high pH) and low in phosphorous. Adding bonemeal and rock phosphate will help.