You need to make sure the most advantageous partners, and the best talent are available. But be sure to develop your selection criteria based upon customer requirements and issues, not just counterbalancing strengths and weaknesses of the teaming organisations.
For larger projects, funding is easier to obtain early, before funds are committed to other purposes.
Customers require time to understand and accept your positioning of your team – so plan for that process and time as well.
Lastly, you need a signed agreement to collaborate freely. Teaming discussions take time, involving senior management, attorneys, and formal contracts.