Well, since you seem to be pretty satisfied with your existing grass I would suggest overseeding with some type of clover, or a complimentary type of grass. I know nothing about your particular conditions or climate, so I would further suggest talking to your county extension office or possibly the agronomy department of a major university in your state. As far as fertilizing goes, I believe it is usually done in the spring. You would need to take soil samples and have them analyzed to know what your soil is lacking. As to spraying weeds, I would suggest you check into using a pre-emergent that targets your particular problem area. The advantage of a pre-emergent over the usual repeated spraying method is that a pre-emergent prevents the weeds from ever coming up, thus giving the desirable grasses/clover/alfalfa/etc a fighting chance of thickening to the point that they choke the weeds out. I hope this helps. :)
