I'm a full-time freelancer/contractor but I did a lot of on-the-side freelance work when I was permanently employed years back. You need two things. 1. Your daytime job needs to be relatively easy and low-stress so you have surplus mental capacity to work more in the evenings and weekends. 2. You side gigs need to be simple, repeatable processes. I used to do things like custom email newsletter and blog design, which I could easily churn out using common templates. Pay-by-the-hour services such as SEO and PPC management can also be pretty lucrative.