DOs: Advertise. A lot. Door hangers, fliers, etc. Do a great job and be personable. Sell yourself, not your services. Do try to keep your jobs in the same neighborhoods. Don't drive all over town to mow one lawn then another. You'll eat up a lot of time that way w/ nothing to show for it.
DONTs: Don't lowball. Price competitively, but there's really no need to go below market value. There's more work available than you could ever do yourself. Don't do average quality work. Go above and beyond what you see others doing.