We started looking for affiliate software when referrals became impossible to track by hand.
While it was just a few people sending users our way, a spreadsheet was fine. Once we had partners asking about links, coupon codes, and recurring payouts, you can guess we quickly realized we needed a real management platform. We're still a small team, so I really didn't want something that would turn into a whole partner-ops project.
I looked at a couple of options. One seemed built more for bigger SaaS companies with resellers, co-selling, partner marketplaces, and a proper partner team behind it. Another looked decent for subscription tracking, but felt a bit heavier than what we needed at this stage.
We ended up going with Rewardful, it seemed to fit how we actually work (and it turned out that it did). We use Stripe, so setup was pretty straightforward. The main thing I needed was recurring commission tracking, because one-time affiliate logic doesn't really work well when customers are on monthly or annual plans.
My two cents on setting things up - coupon tracking, that's a big one. A lot of referrals don't happen through a link click. Someone mentions your product in a video, adds a code to a newsletter, or sends it to a client later.
I plan to reach out to a number of clients and turn them into affiliates, offering discounts/upgrading to the next tier for the same price. Also looking into listing us on a couple of directories. Basically, now that we have a system in place, we want to slowly expand.
So my best advice would be to start with a small group of affiliates, and try to have them engaged/offer them support. And once the time comes that you need to reorganize, don't look for "the best x", but the one that fits your situation (be it a platform, strategy, or something else).