Figure out how much more business you want and can handle. You don't want the phone to be ringing off the hook, if you can only handle 10-20% more business.
Try to figure out if you are getting all the free advertising that you can get. I post signs on the neighborhood market for free. I don't get a ton of work from them, but each job that I get is close to home, and their neighbors are close to my home as well. Its free and effective.
Craigslist is free too. Some people will be looking for a CL deal, so make sure to market yourself as a professional, not the discount driveby-treeguy. I get a lot of work from CL, and their neighbors, and their friends, etc.
Some people use the yellow pages exclusively. Some areas have more than one directory. Figure out which is the most used.
Professional lettering on your truck will cost as much as a small yellow pages and will last for years. Mobile advertising. If you are doing high quality work and looking professional, people will see it, and your truck advertising with phone number.
A blog can be easy and free. An opportunity to show pictures and have homeowner letters of reference. It will have your company name in the address, which can go on your business cards. I haven't started using mine yet, but its
www.southsoundtree.blogspot.com pretty easy for people to remember, easier than a phone number for those times that you may be in casual conversation with people, but don't have a business card on you.