In Real Estate it is Location, Location, Location...
In the 25 years that I have been producing signs and graphics, for me it has been Service, Service, Service and Quality, Quality, Quality!!
I have advertised very, very little over the years and really only ever had business cards until they ran out, but I didn't care because my referral business was more than enough to keep me busy. I built a reputation of the two qualities that I mentioned above.
Here are a few things I did to separate myself when starting AND to this day...
1) I, PERSONALLY, deliver every job I sell to the customers door within 20 miles without a delivery fee. (obviously, if they are out of your county or beyond this isn't feasible) This just blows people away and almost always gets them talking about or even requesting something else = repeat business!
2) If there is ever an issue with the sign, label, decal, nameplate, etc., I show up at their door to assist in figuring out the problem. It may be an employee issue or seldom a material problem. Either way it is a "we will take care of it now" attitude.
3) I NEVER deliver JUNK. If you make a sign, I don't care how little or much you are charging, never send it out full of bubbles, slits, wrinkles, etc. This will put you in what I call the "Fast Food" Sign Shop category - It is a bang-it out concept. This is not where you want to be, unless you want to spit out corrugated signs all day.
4) Don't cut corners on materials if it could jeopardize a customer relationship. Intermediate vinyls and materials have their place, but using them for the purpose of being "cheaper" than the competition is a dangerous path to take unless the customer knows upfront.
Maybe to "dip" your toes, you might start a relationship with a signmaker a little bit out of your main area and purchase vinyl and signs wholesale to get started before investing in equipment and also to see if you have what it takes to be an OntSignGirl!!
These are just some things that work for me, the others here have some great advice also.