If you mean assistant for business chores like emails and overseeing tasks that do not interest you, get someone middle aged with a spouse & kids who isn't curious about stuff and generally content with life (if they are religious that's a good quality). Another soft skill found in this species is hand written note taking. It's a sign that they're antiquated, take their work seriously and are not lazy. Gather info on them and their family. And never talk with them about the "glory of entrepreneurship" or any crap that will get them thinking, or greedy. In fact, do not talk anything meaningful at all other than instructions on their chores and general polite small talk. This is critical. Rather, stress the virtue of hard work. Pay them well, more than double the market rate, give them gifts they'll like, act like you care for their kids or whatever. Also, act transparent but never tell them more than they need to know, and if you can pretend that there is someone higher up the food chain than you then ever better. Don't admit that all the money is going to you personally (it goes to the company and you just get a salary). Get your admiration induced erections elsewhere.
These subdued yet functional people are just like robots. You program them once and they'll keep doing the tasks expected of them. Don't expect creativity and loyalty from the same creature for long. Thinking is your job. The assistant is (should be) capable of rinsing & repeating the process unquestioningly.
