Yes, there are plenty of people here who have successful businesses. That means they don't have 24 hours a day to hang out in the forums answering questions.
Marketing is not a science. Yes, you can (and should) test things, but there are so many variables it becomes more of an art. There are no hard-and-fast formulas you can use. Instead, most of us end up using trial-and-error to find what works.
So, you tried Craigslist and it worked (once). That might be an indication that Craigslist is a good advertising channel for you, or it might have simply been coincidence. So run some more ads there. It's not as though it's going to break the bank. Do you continue to get a good response? Then keep running ads. Do the results disappoint now? Then maybe the first excellent results were a fluke.
If you're not getting results from door to door and flyers, maybe those aren't the optimal way to reach people who need computer services. To me, those seem like things that would be good to reach individuals. But if you really want to be successful, I think you need to be reaching out specifically to businesses
They're the ones who have more computer gear, and more urgent needs to keep that gear up and running... and more need for design services.
For instance, how about reaching out to small business owners where for a (reasonable) monthly fee, you'll be their on-call go-to computer repair person? Many months, they may not need any help at all, or just some basic telephone support, so you could service a relatively large number of businesses. Plus, by having them on monthly (or annual) contracts, you would have a more predictable, steady income stream.
Thing is, you aren't going to impress business owners with flyers and door-to-door. They're busy people. You want to show that you're a "real" business yourself and that you respect the value of their time. So first identify businesses in your area that don't have someone on-staff who does what you do.
Then you want to contact them, maybe through LinkedIn? Maybe via a snail mail sales letter or other direct mail piece? Just a brief introduction. Tell them you'd like to talk more with them about their needs and how you might be able to help them save time, save money and run their businesses more efficiently.
And tell them you'll be following up. Then follow up with a phone call and try to set an appointment -- ask for a half hour of their time, perhaps.
Talk with them. Listen to what their pain point actually is. Do they need to keep older computer equipment running for as long as possible because they don't have the budget to buy new? Do they need someone to help them identify what new equipment to buy so they don't waste money? Do they need help with design projects? Do they want someone they can trust to call on when they have a computer emergency? Can they fix their own gear, or do their own design work, but find they're spending too much time on that so they can't work on other aspects of their business?
Whatever it is, show them how you can help.
And as you get one client who's happy with your work, ask them for a testimonial and for referrals to others who they think might benefit from your services. Offer them a discount or "finders fee" for every referral they give you who actually signs up. You could even give them brochures they can give to their colleagues that give the referral client a discount, too -- so they can be the "good guy" who gets discounts for their friends. Repeat with your second client, and your third, and so on and so forth.
Organic search results from Google and other search engines should never be your primary marketing channel. You want to devote the bulk of your time and attention to channels where you have some control. Remember, when it comes to search results, what Google giveth, Google can taketh away -- with no advance notice, for any reason, or for no apparent reason at all.
You do NOT want to spend all the time it takes to build yourself up in the search results, come to depend on the traffic they bring, and watch them all go away in an instant because Google's database had a hiccup, or they decided to change their algorithm to discount one of your primary optimization tactics.
Best of luck with your marketing! Please feel free to keep asking questions -- just have a little patience about getting an answer, please!