I know many people find guided meditation helpful; we don't tend to like it personally but it can be helpful for some people in focusing on calming imagery.
One of our alters does meditation while exercising/doing yoga/etc, it can make internal imagery easier to focus on I think.
As far as entirely internal, having calming mantras or something external to focus on can help sometimes. For example, imagining light going from the top of the head to your feet can be easier than imaging an entirely new situation that's also comforting to you. We've also been finding internal dialogue to be a helpful mechanism in expressing our feelings to each other and making sure everyone is heard. I do know that can be difficult though; we've only recently started to be able to communicate and we were told we had DID for the first time in early 2020.
Any sort of physical explosion of emotion seems to be helpful as well, in whatever that means to you - jogging, talking it out, art, etc. Within healthy mediums we've found this to be useful.
These are just some mechanisms that tend to work for us, obviously everyone is going to vary but I hope maybe some of this is helpful