although i do not have arthritis my problem with using a dslr is i lost most of my right hand in a work related accident some years ago and it left me with most of my thumb and not much else for a right hand it's a shame they don't make a left handed dslr it would make my world a lot easier
so i use a cable release ( remote shutter cord ) and it helps no end
another thing i have tried is a pistol grip but you don't see these too often the last one i managed to get my hands on was made by HAMA but i had to modify the trigger to work with canon dslr's not a hard thing to do and works very well unfortunatly now i have moved on from the 300/450d type and on to the 40/50d i have to modify the plug to suite ( just havn't got round to it yet )
a monopod can be usefull in some cases again using a cable release it allows me to hold the monopod and press the shutter ( cable release ) at the same time
there is a wealth of things to use to keep the camera steady using a bean bag under the lens for example whilst holding the camera with whats left of my right hand and using my left hand to hold the cable release and resting a long lens on the bean bag for hand holding so to speak
bit long winded but hopefully an insight for some...