6 year old DS has always sucked two fingers on his left hand. We decided to invest in a fingerguard and DS has been wearing it constantly for 2 weeks (we just took it off every night for his bath/shower and also for swimming and judo) and it seems to have worked.
Last night DS didn't suck his fingers at all as far as we could tell and I checked on him about 73 times (okay, maybe only 10 times) and his teacher said she didn't see him sucking at all at school today and tonight again, no sucking.
So, i know it's very early days but it looks like it might have worked. I hope so because DS so wants to stop too.