Increasing rental income is the best way to increase yield, but you need to give tenants a reason to pay the rent you’ve attributed to the property.
Larger investments can help tenants save money in the long-run in terms of gas and electricity bills, so they may be more willing to pay an increased rental price. However, it’s important to note that you can’t increase the rent at will and must abide by the terms set in the rental agreement with the tenant.
If you are expecting a void period, why not take advantage of the empty property to renovate or add extra rooms? That way, you’ll make the property more attractive and improve rental value and therefore rental yield.