You probably would prefer advice from people who are more regular with it but here it is anyway. I just try to block the "do I want to do this or not?" thinking phase. If I leave it to myself, I don't really want to do anything, including the things I enjoy so blocking that train of thought and refusing to spark a discussion with myself whether this is optional or not helped the most. I just pretend it's something to be done, end of. I think of something else as I get up to dressed to go on a walk. (Or do a short workout, whatever you want to try.) Brain is a toddler ready to throw a tantrum in my case and once I give up dominance by deigning to negotiate, it's a lost cause. And 9 times out of 10, actually starting to do it is the challenging part. Once you begin to do something, you can carry on as long as you're comfortable. As you keep doing that, that initial resistance becomes weaker and weaker until it really finally becomes something you just do and the mind doesn't protest at the thought of doing something unusual. It just becomes normal.
It also helps to break the activity down to the absolute bare minimum. Don't sit and think "I should do a workout/go on a walk". Just say to yourself, you'll turn on a workout video and stand up. That you'll put on a coat and step outside the door. That you'll stretch your back. Once you get that done, again, most of the time the initial resistance breaks and it is easider to carry on. If you still don't feel like it, break it down to the next smallest step you can imagine. Walk to the corner and return. Do the first 2 minutes of a workout. Do 5 crunches. Whatever you feel like. As you build, you'll find it more managable to "just do it".