Those are also really good ideas. You're definitely right; I need something special that I can associate with magic that is portable (and so I'll be able to take it to France with me this year). And while I don't find visualization very useful, there are images that inspire magical feelings for me, so I'll try to collect more of those. Thank you!
One thought that's sensation based - one thing I did when starting out was to be very attentive to how I had my (long) hair when I was about to do ritual. Normally, I wear it up/braided/something other than loose, as otherwise it catches on everything, but for ritual, I'd wear it down. You can get the same basic principle with anything that will feel different to you - a braid at the side of your face, a piece of jewelry (heavy enough to be noticeable: very light-weight stuff doesn't work as well for this purpose), putting on a light shawl, whatever.
The other is to look into physical positions as a way to trigger your mind to go "Ok, working on X now." Again, not always suitable for all purposes, but can be really helpful, even if you start by doing whatever, and then move into other positions depending on what you need to be doing.