Rather depends what's causing the shade - if its overhanging large trees, then the soil will probably be dry and depleted. Regardless of the source of shade, the best you can do is to dig in organic materials, such as your own garden compost, composted animal manures from the garden centre, or other soil conditioning composts that they sell. You may need to do it every year if the soil is really poor, but if the problem is too much dryness at the roots of your plants because they're competing with large tree roots, it won't cure that, although it will help the soil retain a little more water.

in addition to Bamboo's comment: at this time of the year I put a 2" layer of compost/ leaf mould on the soil and let the worms take it down for me. saves digging. the leaf mould is from beech leaves that fell 3 yrs ago. the mulch goes at the base of the beech tree.