At the end of the season, cover the whole garden with black plastic. Leave it on for at least 3 weeks. This works best if it is sunny and hot, but will work any way. The plastic blocks the light and kills any existing weeds, and if it is hot enough, will heat the ground enough to kill many weed seeds. You could leave the plastic on until you are ready to plant the nest year. You also need to be careful with compost. If your compost has not been heated thoroughly and you put your weeds in the bin, the weed seeds will still be viable when you compost your garden.