My wife and I started out as payroll and bookkeeping employees for contractors and moved over to being a business ourselves, servicing our former employers. Many of the businesses we serviced had employees that eventually went out on their own, so we serviced those businesses. All those folks gave us referrals. I've gotten hundreds of small business clients over the years. (btw I sold the practice I'm talking about here.)
These are the other things that worked for us over many years. We used multiple approaches and put more money into anything that returned good clients.
1. Get good at Quickbooks and promote bookkeeping work. You won't make a lot of money at it but it can keep you busy in the off-season and you'll be in the running if the owners want someone else for the tax returns.
2. Promote for and do a few "CFO for the day" gigs, this will get you in direct contact with the owners. Yes, I go sit in their office and pay their bills, clean up their QB 2 afternoons a month. I got $50 an hour and wound up with a lot of the tax work for people associated with that business. I only do it for people I really like and that I think will make great tax clients. In this way I got paid $50 an hour to learn QB. (I already knew accounting, I just didn't use QB.)
3. Befriend as many freelance bookkeepers as you can. Answer their questions for free and help empower them at their gig, and they will reward you with business clients. I always took premium care of my bookkeeper friends and they brought me great tax clients. See if there are any meetup groups for QB in your area.
4. Get a mailing list for new businesses and do mailings to them. This will get you some interviews. Some areas have great new business mailing lists, some are terrible -- always get a free sample and go drive the area. A mailing list of new corporations and LLC filings can be very fruitful The post office in some cases will now let you specify an area to mail to (such as a street that is mostly small businesses). Mailing can be expensive, we had ok experience with professionally designed, large color postcards. The message should be carefully designed (that's a whole other topic.)
5. Join multiple Chambers of Commerce. My luck with getting good clients from Chamber meetings has been terrible. However, I use it to work on my spiel and get used to talking to business owners. We have networking twice a week, and business speed networking every few months at my Chamber, and I love it.
6. Make sure you meet as many of the business owners around your office and your home that you can, and make sure they know what you do. Same with landlords, vendors, contractors for your home or office, etc.
7. At every church or group that you belong to, make sure the other members know what you do.
8. Ask your existing clients for business referrals. If you just do it, it will become smoother. Our quarterly newsletter promoted referrals as well. Some businesses never gave me referrals, I don't know why. Others would send me multiple businesses.
9. Have a few niches, like HOAs or gas stations or electricians. Non-profits was one of my niches. A few days of study will teach you the oddities of those businesses and now you're more of an expert. Mail to more of those businesses.
Of course, know what you're going to sell them. I had several packages. I prefer an annual package for several thousand dollars, paid annually or quarterly. I did payroll but I wouldn't recommend it. The last few years I was in business, I wouldn't personally do any bookkeeping, I would sub it out. I never touched books that were completely messed up (I saw a lot of that) but told the owner we needed someone else to come in and get it straightened out. I would supervise only.
Anyway, that's a long answer but that's how we did it.