maven, try drinking more in the morning/early afternoon so you are fully hydrated before bed time, also fizzy drinks and coffee can irritate the sphincter/bladder and so cause involuntary urination, try to drink plain still water as much as possible at least that is what i was told when my problems started. also make sure you fully empty your bladder just before going to sleep.
you can get a waterproof matress cover pretty cheaply, but in my experience if you can afford it it is worth getting a good quality one unless you like sleeping on a plastic bag!
if you only leak a 6inch circle every few months it may be worth wearing a pant liner in you knickers to bed, if this turns out to not be absorbent enough try a night time pad for periods, if that is not enough you can get more absorbent ones in the chemist specifically for incontinence which hold up to three full releases! unfortunately i am double incontinent so use pads that are more like kiddy nappies...very sexy!