Yellow pages can work well. Do something different helps like colors or reverse colors. If last year was really bad due to heat or drought or insects and lawns died you can try to target your ad to address this but it's real tricky. Start small and slowly get bigger and track how many calls come in etc.
Also having an ad in rich ($) towns or towns which have more of a need for your services or developing towns can work well.
As more and more people get Caller Id's telemarkiting will be less & less of a means to contact. hope this helps