You should always aim to promote the positive aspects of the opportunity to make it attractive to a volunteer. At the same time it is very important to include all relevant information and to ensure that it is up to date and accurate, locations and equipment or clothing requirements for instance.
Only limit the eligible volunteers by requiring a DBS or INDUCTED status or minimum age if the opportunity really does warrant it, the more volunteers that can access the opportunity, the higher your uptake will be.
If you are part of a linked network of volunteer programmes then consider making the opportunity available to the wider group to increase the chance of finding the right volunteer.
Take your time and give as much information as possible and always remember that these are volunteers and they need a reason to give up their time for free, and volunteers always love a perk!