I hit my first plateau after the first 15 pounds. I reassessed my calorie intake and my exercise routine. I adjusted my calories down about 100 to 200 per day. I added resistance training, and upped the mileage per week that I was walking by a half of a mile for my long walk, which is once to twice a week.
I'm training for a 5K, so it was time to up my mileage anyway.