Offer market pay. "Smart" people are smart enough to know their value, and unless you're looking for a cofounder you need to pay properly.

If that's met, just do some advertising - I've found LinkedIn is useless, indeed.com gave better candidates, I'd try other "real" job sites like that.

Depending on your needs, consider part time as well which may be more attractive to some people.

One thing I did that worked was call my former university professors, they may know good fresh grades, postdocs etc.