The success of a project depends on taking the right direction and that requires input from people who have specialised knowledge in the domain in which the project is being undertaken. Also you don't want to reinvent the wheel if the knowledge can be accessed and used by engaging an expert.
The key is to identify and engage right expert or sets of experts. Also they need to be engaged at right time otherwise engaging them to do post-mortem of the failure of the project is of no use. The key to the success of expert judgement is to identify the right expert. There is no dearth of self proclaimed experts in every field. Just don't depend on how good a speech they give, look for what they have delivered in the past. The challenge is to find the authentic one for your set of problems.
- First focus on what skills/expertise you need. Once you know your requirements, getting the right expert becomes easier. Don't first get an expert on something and than try to map to your thing.
- Getting an expert through reference of someone whom you know. Ideally the expert has advised your reference for the similar set of problems/domains. If someone is expert on river don't assume they will be expert on sea, though both contains waters.
- Check the credentials of experts. Internet can many time point you to the engagements that the expert has done in the past.
- Ask for reference from expert that you can check on. Ideally if possible try to reach to those references through your reference to get a more unbiased opinion.
- If the success of the project significantly depends on the expert, than first engage them on smaller assignments to get a feel of things. Once both the parties are comfortable, sign the big contract.
Once you are sure of that the expert is the one that you want to engage, the success also depends on the mechanism of engaging the expert. Expectations and mode and set of deliveries should be outlined clearly in the beginning. For significant engagements, conflict resolution mechanism should be in place and conditions under which the contract will be terminated.
Be very clear of the inputs and outputs from the expert. What data/information/artifacts you are going to make available for the expert and how the expert is going to deliver the results?
If possible and if budget allows, it's better to have more than one expert on board. The voice of dissent uncovers many facts and fictions.