Whether your soil is light and sandy or heavy clay, digging in bulky organic matter - that is well-rotted manure, compost, leafmould, composted bark, mushroom compost, in fact anything that you can get in bulk - will help.
In light soils this provides something to hold moisture and nutrients; in clay soils it helps break up large particles so the soil doesn't crack, drains better and is easier to work.
Digging sharp sand into clay soil may improve the structure and make it easier to work, although on a very heavy clay soil, just stick to well-rotted organic matter, as adding sand can make the soil even harder.
The best time for soil preparation is in autumn for clay soils and spring for light, sandy soils. Autumn digging lets the soil be further broken down by frosts and rain. But do it whenever you can. Start by digging over the soil to a depth of 37.5-45cm (15-18in) and incorporating your organic material as you go.