Test the soil in several areas to determine pH -- use lime to help bring it
in line but get it down now for next spring. The best thing you can do -- a
light dressing of bloodmeal to increase nitrogen, dressings of compost 3-4
times a year, keep an appropriate mowing schedule (3" high, more often in
rainy periods, less in dry periods), mulch the clippings back into the lawn,
and reseed in autumn and spring (I also do a seeding just before summer heat
starts to hit). Foster healthy soil and you will not need chemicals (which,
BTW, will do more damage than good).

James