I have had mine for so long I forget when I bought it, in that time it has been on plenty of vehicles and I have yet to find a time when I forget my registration, or give the last one.
However, I did put it on retention when I bought the AMG in 2005. After the first 2 or 3 returns to get the damn thing fixed, I bought a 2005/55 plate for it so that selling it would be simplier. Then it was just a matter of actually breaking the bond with the car, which took another 9 months and 3 more breakdowns.
At the time when I was running 2 cars, it was easy to remember which car was which, and I have several work runabouts at the moment, which need me to remember the number plate when filling them up. (So I take the right card for the car)
And now the car its on, really does look daft with a private plate! and that makes me chuckle too. In fact, around Dec 2008, the car will be worth less then the plate or sooner if I have got the plate market a bit out.
It has my full initials on it and is 100% me. its non year dated too. So if ever I end up buying my dream car (A Bentley speed 6 Circa 1929) it would suit it.