If you're starting to get frustrated by your child's lack of success in mastering the potty, or if your child seems to be actively resisting potty training, it's a sign that you both need a breather. Soldiering on because you're anxious to toss the diapers or because he needs to be trained to enter preschool may make both of you feel too pressured, which will only sabotage your efforts. Wait a few weeks and revisit the issue when you've recharged your supply of patience and your 2-year-old seems less negative.
You may also want to ease up on the potty-training mission if you notice that your child's reluctance is accompanied by constipation. If so, address that health problem first by varying your child's diet and getting him to drink lots of fluids. Talk to your child's pediatrician if you have any questions.