I haven't socialized too many adults, but Lila was my kitten that I raised like a puppy. I crate trained her (we played "Crate Games" by Susan Garrett!) and she knew what "bed" meant, she'd do a flying leap into her bed. I took her for (crated) car rides, took her to work and let people hold her. She was about 5 months old when I held a Christmas party at my house and we played "pass the kitten", everyone held her. She's very personable. She also knows sit, and sit pretty and it's totally cute.
I think having a special roomy carrier is a great tool. You can take them anywhere and they can choose to remain in there and observe, safe from anything they're unsure of. Socialization is more exposure (let them see, smell, and hear) than interacting, especially at first.
I can’t take any credit for how most of mine were socialised. They generally came to us as adults. Hadji was about four weeks old when my mother rescued him - he was from an inner-city feral colony and was abandoned in a car park. No idea how he got there. She did most of his care, and he grew up with two adult cats, and a year later two little sisters joined him from a shelter, Freya and Caitlin, who were four and three months old. They were all brought up the same. Freya and Hadji were sweet but shy and nervous. Caitlin, my Katie, was sweet and afraid of nothing, an in-your-face cat all her life.
I'd have to say our last cat, Neely, was the most challenging of all the cats we've rescued. She was an owner surrender which should tell you something but we adopted her anyway and gave her a forever home. We're sure she spent time outdoors by her behavior. She was a force to be reckoned with once we had her treated for a bad URI. She was a mischief maker, a night owl, a true diva who did not want to share our home with any other cat but befriended our dog as her playmate. It took awhile but eventually she became very affectionate towards me and slept beside me on my pillow.