It depends for me on the situation I'm needing to ground myself in. Some of the things I do (or try to do) have already been mentioned. I say try, because I'm still quite rubbish at catching myself early on, remembering in the moment that I have these tools to call on!
Two things that haven't been mentioned, but which work or me, particularly on intrusive thoughts/memories and if I'm getting visual flashback stuff going on...
The first, and the one I find most effective, is to draw a square in my head over whatever else I am seeing there. Start in one corner, draw the top line, when you get to the first corner count to 4 then draw a line down for the first side, again count to 4 when you reach the corner, repeat for the bottom, and going up the last side. Then keep going round, counting to four at each corner, until you are more settled. Once I'm a bit more settled I might then be able to do something else like spotting things around the room etc, but I find that I really struggle with external type grounding tools until I've actually dealt with some internal stuff - not sure how much sense that makes?? I need to meet things like for like, so if it's images in my head that are disturbing me, then I need to tackle them with something inside my head too. If it was some external trigger perhaps, like an object I see that might set me off, then I am more likely to protect myself for looking for other things in my environment to distract me.
The second one that I use quite a lot, again and internal in my head grounding, is to freeze the unwanted image in my head as if on a screen and then shrink it, replace the image with one that I want, and enlarge the screen again. (I have a couple of fairly simple images I use for this. These are from photos that I have taken and processed/edited myself, which helps to lock them in my head and also gives a different association for me because I am able to draw on the feelings from the time that I took them). I have simple one or two word phrases associated with the image as well that I can repeat in my head to help with this one. I'm supposed to touch my wrist or my thumb when I'm bringing up the new image as well and in theory that creates an association you can use to bring up the image at a later point by touching the same spot again - I'm rubbish at remembering that bit though!
Photography itself helps ground me quite a lot actually, both the taking and editing of photos. Taking them helps keeps me connected to the present moment. Editing works as a distraction.
