Yard-signs work well, it lets your work sell it'self.
Offering your clients discounts or
gift cards for any sales from leads given.
Offer the local barber finders fees for any sales from leads, or a few bucks for every lead and a percentage from every sale.
As I have said many times, the revolving accounts are what will keep you going: easy math says that
a 300 client portfolio
that averages $1000/visit,
and is on a three year cycle
generates $100,000/year.
If you get a lead for tree "A", give line item bids for "B" & "C". If there is a small tree near by, lump it in with tree "A" at a discount.
Small ornamentals are a great way to turn a one shot customer into a long term client. I know many properties where crabs are pruned every year or two. "I can do the crabapple and foundation shrubs for $250"
Wave to the neighbors as they drive by, if the slow down to look, ask if you can help them.