For improved water retention I take out a trench and fill it with rubbish wood and half rotted compost, then put the soil back on top. It works both ways, drainage and water retention, you can pick up old wood in every layby for free, and as it decomposes it releases nutrients that are wicked back up. I am a cheapskate mind, so it is the free bit that mostly appeals.
PS Good to see you posting more, welcome to the forum. What size of garden do you have? Do you grow flowers or veg., or both? Always nice to hear personally from people.