Green manures are short-lived crops which you dig in at the seedling stage. They add organic matter quickly and increase both the soil's fertility and its moisture-holding ability. You will notice a real improvement.
Most seed companies sell green manure seeds of various kinds. The best are the very quick-growing such as mustard. The packets may look expensive at first sight, but remember you are buying a huge amount of seed, not the usual little packet. However, in the case of quick-growing plants like mustard, you can save money in future years by growing your own seed. Sow some of the seed in a corner where it can be left undisturbed, and let it run to flower next year. Save the seeds when ripe; the pods will turn dusty beige. Don't let them go too far or the seed will be shed where the plants are growing and you won't be able to collect it all.
For more about green manures from the RHS, click on the picture.