A skills audit around the Trustees is a good beginning – you will quickly establish any gaps in the key areas e.g. legal, financial, increasingly HR, business management and any very specific area requiring a particular skill set.
Then begin the process of filling those roles you will have identified being very careful not to increase the Board to an unmanageable number. So it might be necessary to look at the rotation policy to enable the necessary introduction of new Trustees – no need to be sinister in the approach, only to be honest about what you are trying to achieve.
Some roles might need Terms of Reference, e.g. the Chair, Treasurer and maybe Legal – in that way it enables not only the management of expectation from these individuals but also allows for effective succession planning.
Be mindful of your governing documentation around the number of Trustees you might be allowed and any other caveats that might be buried in the detail.
Above all do not isolate those roles you may have identified to achieve the necessary balance – the Board is a collective and must act in a cohesive way at all times, the role of a strong Chair I would suggest!