Here are some of the tips that could help during planning poker estimation sessions
1. Ensure that all developers show the estimation cards in one shot. If the cards are shown serially chances that it could lead to "estimation influence" factor.
2. Don't sit in crumpled space during this session. Try to use a large room, and people sitting in such a way that they can see each other
3. People who have no idea of use case or requirements can opt out of the session. They need not be forced to be present.
4. Have a spreadsheet open and the computer connected to a projector ready. This would help all the team members to see the requirements clearly.
Please note that, Planning poker estimation, even though more light weight and accurate than other estimation techniques, it could lead to in accuracies. The inaccuracies results from inadequate estimations in the technology or domain that they are working on.