Example: A new sitter is coming over - I get a picture of her and tell him her name over and over. I have her meet him prior to an actual night where we leave, so he feels like he has seen her before and knows her. The meeting tells me if his and her personality will click and it did.
Example: We are moving. I take pictures if I can of the new house and show them to him so he knows what to expect and it seems familiar once he gets there.
Example: Leaving the house. I have found transitions away from the computer or TV mean at the end of a level or program is the best time to stop so I indicate he must stop and if I ask him to stop he will with extra oversight. If we are getting shoes and socks on I shut all lights off in the house and electronic items so the focus is on the hallway where the shoes and socks are. Like a moth, he goes there and then the focus is to ready to leave.
Example: Oven Timer. For some transitions, I will set the kitchen oven timer and he knows when it goes off that is the indication to stop and move on to the next item. This only works if I give him a small amount of time first - 5 minutes until the transition; and if I am ready to go.
If I am not ready to transition, he will take advantage of my un-focus and then the transition does not happen until the next firm opportunity. It is not always easy to be ready, but aren't we all just doing our best? Keep it up! and stay patient.
One thing that I believe helps is by talking to my son about the change and giving him as much detail as possible. Also I might show him pictures or bring him by the new place. For example when he was transitioning into the school district for services, I brought him to the school and play in their playground and walk around the field for a few weeks before he started. I also started talking about his new teacher and used her name so he would have some familiarity when he got there. I was able to get him into see the new classroom before school started which was not planned, I met the teacher to get a copy of his IEP and she brought us into the room and met my son. Definitely picture schedules and just conversation is a good place to start.