Heh, I still remember my first time tutoring. It was quite the flop. The kid didn't even contact me back for a second session. But yeh, obviously I've improved much over the years and it does come down much to experience. But generally, what I've learnt over the years is:
1. Know your stuff. Revise if required and don't try to just wing it as I initially did. This is a given and really your responsibility being a tutor in the first place.
2. Preparation goes a long way, so notes and Powerpoint slides and such will make things easier for both you and your student.
3. Be confident. Even when you're possibly not. As long as you're confident, people will generally accept whatever you say. Given this however, if you're really not sure, just say so. Students don't expect you to know everything as a student tutor. Heck, even teachers say they don't know a lot of the time. Just make sure you know where to find out, and let them know that you will look it up.
4. Aim for knowledge retention by your student; don't simply throw a heap of information on to them. This is possibly one of the most important things as a tutor. For something like psychology where I assume there's some lists and definitions to remember, don't just keep ranting to them. Provide mnemonics, make them repeat the definitions and such. Next week, revise what you did last week quickly to make sure it's retained. I have a feeling that these sorts of memory techniques would actually be part of the psychology syllabus, so basically, practice what you preach I guess.
5. Be structured in everything you do. Structure helps both retention and understanding. Let the student be aware of the structure you'll be taking by essentially starting broad, and then going more specific afterwards.
6. For something like a science, simplifying information is important. Give analogies and so on. Although you want to give the impression you're smart and whatnot, don't just go in there with the idea of simply impressing the student with random facts and minor technicalities. Give them what's important first, make sure they understand it, and then perhaps move to the more obscure stuff if you must.
7. Cater for every student individually and have a fun time teaching. I would never have survived doing the same 3.5 hour lecture a good 60 times if I literally just said the same thing each time. Treat every student differently, make some jokes and just try to have fun in general.
Given that I've actually taught you, hopefully you can relate to some of these in your experience with me. Anyway, hope that helps.