Understanding the value you can add to a board is the first step. When looking at opportunities it is also important to look at the overall composition of the board to ensure you will have the right support if this is your first role.
It is also essential that you have spent time with your prospective fellow board members, getting to understand the culture and ensuring that you will be able to thrive in the environment; remember not all board roles are for you and that is ok.
In addition to the above, do consider the increased stakeholder scrutiny on boards, and what the result of this is in terms of expectations; professionally, standards of practice needs to be consistently high, with personal time commitments being considerably more than in the past. There are also statutory and legal responsibilities to consider.
Finally, it is vital that you have conducted your own due diligence on the organisations you plan on joining. Interviews go both ways, and a board portfolio should be proactively designed, not simply a collection of ill-considered roles. You have built up your professional profile over many years and joining the wrong board has the ability to cause damage to your reputation.