Previously I tried out a referral program, and the results were alright, but not spectacular. I will need to work on the details more before giving it another go.
In the meantime, I'm experimenting with another approach which has been showing some promise — running a special promotion. Basically I'm limiting supply to create demand, by offering my logo design service to the first 20 clients at a 50% discount for $150 (from the usual $300), which includes 3 initial concepts, the source vector files as deliverables, and as many revisions as necessary until the design meets their expectations.
I started last week, and have landed 6 projects so far using this method. I'm extending this promotion to Indie Hackers as well, so if you're interested, feel free to message me for more details. Cheers!