The first thing you do is sit down with your boss and work out a priority system, so that you can set the priority yourself on 75-80% of the tasks if not more. Design a report to let him know what you are being asked to do and give it to him daily or weekly depending on his needs. It is up to him to make sure that this old work goes elsewhere if it is affecting your ability to do the new work. He needs to see in writing how much of it you are being asked to do.
Next you communicate the priority to the person doing the asking and tell them that if they need a higher priority they need to contact your boss. Then only change the priority if the boss tells you to change it. I would suggest you have a priority list visible on a whiteboard, so that people can see why you are not working on their task.
If the priority on a task is lowered due to something else of higher priority coming along, make sure to let the requester know.
It is probably a good idea to have discuss having a time period each day to work on the lower priority work. Then you need to have the self-discipline to stay within that time window. Discuss when would be the best time to do that with your boss. Depending on the support need, first thing in the morning might be best or at the end of the day. But it is critical that you stay within the time period allotted even if the task is interesting or you are not done yet. The other work is your higher priority, never forget that even when it isn't the most interesting task you have to do. If the work is taking more than the allotted time, you need to push it up to your boss whether he is busy or not. He needs to know that you are being over-scheduled.
Next you have to train your customers to understand the new priorities and one of the best methods for that is to change how requests are submitted. Never accept a verbal request of any kind. Period. If they don't put it in a ticketing system (highly recommended) or at least in an email, you didn't get it. You have to be ruthless about this. People will take advantage if you let them.
You also have to be ruthless about not working overtime on a task unless it is absolutely urgent (i.e., people can't log in or the production server is down type of thing). People will try to guilt you into working two full-time jobs. Don't take the bait. Trust me when I say that no one will remember it took two days to do something that you used to do immediately a year from now. They will remember if you don't get your main work done.
You have to mentally consider that this is a change of job position, just like you have moved to a new job at a new employer. You can't do the old job and the new one together, so you provide only the minimum that must be done on the old one and concentrate your motivation and personal investment into the new one.