Remember that this industry is a little different from your duct cleaning business, in that it's both more stratified and more competitive. Duct cleaning is pretty much a homogenious product (AFAIK, which isn't much
), and the market is still growing in its infancy (relative to lawn care).
Lawn care, on the other hand, is more mature and runs the gamut from an individual, self-employed Mexican - paid by the homeowner in cash, will net about $8/hr, and possibly an illegal immigrant - on an itty bitty Craftsman tractor to a full-blown company with company vans/trailers and can do anything you want.
Unless you snag some ultra rich client who doesn't care about price, there is intense price competition in all of the categories. So, first figure out what you want to do: compete only on price, compete on service, or compete on your depth of services offered. Grocery stores that didn't ever specialize in one of the submarkets are getting killed when Wal-Mart moves into town, since they have difficulty competing with WalMart on price, and don't offer service like the small boutique grocery stores, and don't offer the special produce/goods that other grocery stores offer. That leaves them with stuff that everyone else has, at a higher price, with subpar service. And leaves people with no reason to go to their stores.
Likewise, figure out where you want to compete (hint: if you're even thinking of hiring a sales person to work on commission, I don't think you'll get much business in competing in the "price" submarket when you have armies of Mexicans to compete against). See what your competition is currently offering, and what they charge, and run your numbers to see what your cost would be (I'm assuming you probably already did this).
I'd agree that word of mouth from satisfied clients is the best (and CHEAPEST) advertising. Hopefully you have a great client list from your duct cleaning business, and will target them FIRST.
I'd be weary of paying money for on-line advertising, since you will also have to pay to set-up/maintain a webpage. Why not take that time to instead cold-call previous clients and see if you can service their lawn care, or know someone that needs a good lawn care/yard work provider.
Realize that many lawn care companies have a huge array of services: snow removal, picking up branches, trimming branches, planting flowers/shrubs, pruning/weeding flowerbeds, fertilizing, mulching, branch chipping, aerating, weed spraying, leaf raking, retaining walls, patios/walkways, etc. You could even diversify further and offer things like pool care (it'd be a cinch to spend 5 minutes to check their pool chemicals while you're there taking care of their yard), or - as others have suggested - things like powerwashing and pest control. Depending on which submarket you want to specialize in, you might end up having to take on a lot of different equipment and tasks.
Are you personally doing the grass cutting, or hiring others to do it? You'd be amazed at how difficult it can be to find people to work for a little above minimum wage and give a crap about their work. My grandmother is forever complaining about how the people that cut her grass are either cutting her flowers down or not trimming around her driveway.
Check out local municipalities, and find out how they award their lawn care. Normally they do competitive bidding with publicly-opened bids. Might be an easy way to see what your competition is pricing 'large' jobs at. Also check out corporations that have large campuses that need total lawn care. Have any friends that know people that make corporate decisions for lawncare? They'd likely be more able to make a deal based on who you know. However, gov'ts and large corporations will also want things like certificates of insurance...so also consider how much insurance coverage you will need, and how that might impact things. Might be easier to do one large municipality/corporate property than try and land 30 homes spread all around the city (as one poster noted).
Also, it sounded like it's currently just you and the other guy in your duct cleaning business - are you set up with an outside payroll provider? If you remain self-employed, it's one thing...but hiring a couple of people might require an outside payroll provider (if you don't already have one), and might impact your cost equation a bit more.