Giving small reinforcers for each success that they can only have for using the bathroom and then a larger one when they fill up the racks. A small toy from the dollar store or their favorite snack, doesn't have to be expensive.
I just made a chart like this for E, using hers as a template. Looking forward to when her chart fills up so we can get some frozen yogurt or Friendly's sundaes! ;) Toddler Reward System - Sticker Chart
Thomas the Train Potty training chart--I put ours in a cheap Ikea frame. I use dry erase markers to fill in the circles. If he has earned three circles (without complaint or me doing it for him), he gets a jellybean.
Download our FREE potty training tracker and rewards charts to add more smiles to a potentially stressful situation: http://www.parents.com/toddlers-preschoolers/potty-training-charts/?socsrc=pmmpin130618pttPottyCharts