I think a website is a good idea, alot of people use google etc to find local services (I do)
I would like to see;
-Feedback from
real clients, it is obivious when these are made up.
-Any qualifications you may have with regard to the service you wish to provide.
-Insurance details
-Walks available: lead walks, woods, fields, parks etc
-Areas you cover
-Wether my dog would be walked individually or with others.
-Times available
-Pricing, do you discount for more than one dog from the same home?
-I would like to know a bit about you. Is it just you who would walk my dog? What kind of dog(s) do you own? With what breeds are you experianced?
-Wether you are CRB checked. I think people would like to know, especially if you will be entering peoples homes when they are in work.
-Obviously, clear contact details.
I would suggest a professional looking website with a more 'personal' content. Include pictures and stories as this gives a personal touch. You want people to think you business is accessible to them, not to business like but formal enough to show that you are serious about your work.
I don't know about anyone else, but what really puts me off is a website that is not regularly updated. I think it shows a lack of commitment. But that might just be me being a fussy git.
Posters, flyers and signs are also a good idea though. These can be relatively cheap to produce and very effective if know where to put them

This is just my humble opinion, but I hope it helps.