Eric,
yeah i'd say wait till spring for the mail out marketing idea. So you already have a lawn care service? You better go talk to your insurance agent and let them know you're going to be adding the pressure washing service to your current business. This is why: say your lawn care business is insured, and you keep doing what you're doing and in the spring you decide to take up pressure washing, ok so you're out thinking everything is fine and you're pressure washing a $500,000 home, you and your helper are positioning the ladder to get up to the gutters, well since your 32ft ladder is top heavy it makes it hard to manuver, so you and your helper lose control and it comes crashing down into a $2000 window the home has on its 2nd floor.. what do you do... call your insurance company and pray that your liability insurance covers it, guess what they're going to tell you... i'm sorry eric, but when did mowing lawns require you to use a 32 ft ladder and pressure wash somone's house... so that 2k comes back on you... So, make sure you get add your pressure washing service to your current lawn care service before you do any jobs.
About the licensing part: most states do no require you to have a license to pressure wash, but having a regular business license in your city or state means that you are able to say your business is licensed, since you're already licensed w/ your other business there's no need ot get an additional license since this service is basically an addition.
Advertising in the yellow pages: It costs me roughly $500 per year to advertise a business card size add in yellow book. Depending on how big your business is this can help greatly. Consider you get 1-2 jobs from this add and your advertising is paid for... On the other hand if there is 25 others in your local book advertising for pressure washign, it's best to pick a different rout, or make your add stick out by saying you've got competitive pricing + free estiates...
About sticking a sign in the yard:
This can be a good advertising method, BUT most people that have a nice house and live in a gated community dont want people trying to market their business on their property, pulling your truck and trailor up on their driveway w/ your logo's and phone number on the side is good enough.. Oh and if you do do the sign thing, I hope you remove it when you leave instead of assuming your customer is ok w/ advertising on their property.
Jonathan
Brite-Side Powerwashing