Before you start changing your routine it is important to know that garden prep is completely dependent on the conditions of your soil. Things you must consider are: moisture, temperature, organic matter, and texture to name a few. So getting to know what you're working with should be the first step in determining when to till.
Should you even till at all?
A common theme that is appearing in the gardening world as of late is the idea that you should not till the ground at all come spring. The rationale behind this idea is that when you till in the spring time you are disrupting the natural processes that occur in the ground. You are also turning up dormant weed seeds allowing them to grow. The thought is that mother nature does a great job of growing things on her own, she doesn't need you to intervene.
Instead, you should put down layers of mulch and other nutrient rich compost in order to maintain the soil integrity. This replicates what happens in nature and helps protect the soil ecosystem. It will also help attract earth worms who will till the soil naturally for you.
When is tilling appropriate?
Regardless of your stance on spring time tilling, most experts agree that tilling your garden in the fall will be more beneficial come planting season. This allows plenty of time for the organic matter that you mix into your garden to decompose, break down, and become incorporated into the soil through the freezing and thawing process. You will also be exposing harmful insects and weeds to the winter conditions which will kill them, reducing their negative effects. Fall tilling also allows you to plant cover crop which will help reduce soil erosion.
Of course there are certain instances when spring tilling is necessary. For example, if you are starting a new garden bed then you will need to break up the soil for planting. Also if your garden is heavily compacted then it would be a good idea to break up the soil with very shallow till, spade, or broadfork. Using a cultivator could help break up the compacted top soil and allow water and air in.
Give it a try
If you are comfortable with your current method of gardening by tilling in the spring, and are not struggling with weeds or poor output, then by all means continue doing what your're doing. But if you want to try to improve the overall quality of your garden, then get yourself a garden tiller and try preparing your plot in the fall instead of spring.
For more information on fall tilling check out this article on www.tillersdirect.com.