Suggestion, advice on TDD adoption

HI,
 We have recently started using TDD. We have gone through the training, learned some of the TDD good practices/challenges, using junit, mockito tools/framework etc. Since its an early start to the journey, I would be glad to hear some practical advice from folks here, some of the DOs and Dont's, tips and tricks etc. We are going to have a bumpy ride, especially with exisitng code, so it will really help if we can hear from experinced people. 
 

 Thanks,
 pooja