The technique with which I have had the most success when starting a garden bed is sheet mulching. This is not a brand new idea, nor is it something I developed. There are many examples out there to use as guides. I do prefer the method as described by Toby Hemenway in Gaia's Garden. Here it is:
The Ultimate Sheet Mulch
My method of implementing the Ultimate Sheet Mulch:1. Chop any vegetation and leave where it stands. This includes all herbaceous plants. If you can cut the woody plants into small enough pieces that is great. If not you may need to move the larger woody pieces. Don't worry about grass and small surface plants.
2. Breakup the soil with a shovel. Step on the shovel to dig the blade in deep. Then work the handle back and forth to let some air in. Take out the shovel and move to the next spot about 5-7 inches in front or behind (depending on which way you are going). Perform this aeration over the entire garden bed.
3. Water the whole area to get things nice and soaked.
3. Lay cardboard down over the entire area. Overlap at least 6 inches, 8-12 inches is even better to prevent shoots from working through. You can use multiple layers of newspaper for the same effect, but cardboard doesn't blow away as much.
4. Water this layer again until all the paper is nice and saturated.
5. Thin layer of manure if you have it. I've used composted manure, uncomposted manure, from cattle and from horses. I haven't noticed a huge difference.
6. Water everything again.
7. As close to a foot of bulk organic matter (hay, straw, dried out lawn clippings, whatever you can find).
8. A few inches of compost.
9. A few inches of mulch. This can be straw, leaves, wood chips, or any other seedless material.
10. Plant right into it.
11. OR cover with dark plastic for a few months in the sun and let it compost down. When you do plant into after removing the plastic, you will probably need another few inches of mulch.
I think any variation of the above will work pretty well. The key is an initial grass killing layer (newspaper or cardboard), a bunch of organic matter, then a SEEDLESS mulch layer.
Here is a pretty well made video showing the results of sheet mulching.