the age and maturity of the child you are working with is very important.  I have seen many first time parents become nearly obsessed with thumb sucking and potty training.  my experience and studies have taught me that each child has their own time/schedule and these habits although maybe not the most attractive to the parents, are also often a comfort source, a way of relieving stress and most often halted by peer pressure as they socialize with other children.  positive reinforcement is always a great motivater.  good luck, enjoy your child and let them know every single day how special and loved they are.