I've just bought my first ever diesel car, an XC90 and after I mis fuelled a works van several years ago (embarassing but we're all human), was worried about a repeat incident with my own car.
I found a useful gadget, (I hope), on fleabay, that verbally reminds you twice, that your car is diesel, when the fuel flap is opened.
I bought this one but there are others:-
http://cgi.ebay.co.uk/THiNK-DIESEL-a...item562d9ee052
Just search for 'think diesel', if the above link fails to work once fleabay remove the old listing.