In my experience, I usually don't promote someone to staff unless they are super trusted friends or I've known them for years. There have only be a few times that I thought a long-time member would make for a good staff member -- if they're attentive, friendly, know the lore and the rules, and seem fairly active. But even then, I only let someone like that be a mod until I know I can trust them enough to make them an admin.

I've had incidents in the past with people who I naively handed the keys to because I had to be gone for a while, but didn't want to see the site go down because of my absence. When I came back, the site wasn't the same anymore -- the whole plot, canons, etc had been completely stripped, and the site had been scrapped together in a useless effort to stop it from dying. I was so disappointed by what had become of it, I didn't even bother trying to become an admin again, or even re-join it.

Another more recent incident was with a site I opened a few years ago that seemed like a really good idea at first. But, the longer I stayed there, the longer I ran it, the more I felt like the site wasn't really what I had envisioned or initially wanted. My co-admin and I had appointed a mod to help us out, who seemed relatively likeable and trustworthy and active, but the moment we gave her mod powers, she made it hard to want to write there and made the site literal hell. Managing with her, being staff with her, is one of the worst decisions I ever made, because she just wanted to change everything and wanted to steam-roll over everything we originally had planned for the site. Because I was scared of losing her as a friend and/or member and because she had basically sucked my muse for the site dry, I ended up wanting to close down the site. When I told her me and the co-admin were leaving, she practically begged me to keep it open. We had a long dispute about it all, but in the end, I ended up giving her the keys. I regret it now because the site's been basically trashed, had become relatively inactive, and full of staff and members that have no standards. I still have admin powers, so I think about going in and deleting it sometimes, because I am just so ashamed to see something I created deteriorate in my name and become something virtually unrecognizable. However, I don't want to have any bad blood, cause bad karma, or have an angry mob after me, so I just continue to leave to rot in the hopes that the person managing it now will eventually have sense enough to realize that the community she has created is toxic, uncaring, and inactive, and that she'll just close it down.

tldr, the lesson to be learned here is that you should try and ask some trusted friends if they'd like to help out first before advertising for staff. Some people just want the power or the label, and don't actually care about you or the site's progression, and some people want to try the role on for size, only to realize its too big for them. If you really want to, you could make a thread/advertisement/signature message to promote that you're looking for staff, but have them fill out an application or form so that you know what you're getting beforehand, and even then, my advice would be not to give them admin powers right away. Start them off as a mod for a few months and see what they're capable of, how active and suggestive they are, how much they want to help, if they help, etc -- like a trial period. At least, that way, if they break a lot of rules, are mistreating people, not being active enough, causing trouble, etc, and they continue to do it after being talked to about it, then you can take their position as staff away and ask them to step down while you look for a new candidate.