Be realistic. Often hard to fill positions stem from unrealistic expectations of the hiring manager. They may insist on hiring someone who is willing to take a 50% pay cut, or a professional step-back, or a lateral move. The reality is that most quality candidates are not actively looking for a job and would only make a move if they are offered more responsibility, prestige, and/or money. In this case, we sometimes have to work internally to align expectations for the role with the talent marketplace.
Know your target talent market. We always begin by fleshing out candidate personas to ensure we know exactly who we are looking for. We try to fill in as much detail as possible in this process such as where does this person live, what are their interests, who are their colleagues and friends, what do they do in their spare time, what jobs have they held in the past, why is this opportunity appealing to them? We may have 4-5 possible personas for each search. If you are having trouble filling a role, it may be that you simply have not gotten to know your target talent market well enough to appeal to them in a resonant way.
Get proactive. Once you know who you are looking for, don't wait for them to find you, go out and find them. Develop a list of people who fit each of these personas and proactively reach out to them using tools like LinkedIn. Talk with each of them to hear their thoughts on the opportunity. Be open to their suggestions for possible candidates and test the waters to see if they might be personally interested as well.
Never stop recruiting. A search is not over until the candidate accepts. Never stop cultivating a pipeline of talent until the very end. We even cultivate pools of talent before an opening even arises by forging authentic relationships with top talent in our sector all the time. Making those connections, via phone or in person, and building those relationships with people really seems to make a difference. This takes time, much like cultivating a potential donor but I think it really pays off.