Procurement Best Practices

Hi there! I'm currently developing an internal procurement guide for our in-country teams to help them with buying services/items that they require. I'm trying to come up with a list of procurement 'best practices' and I've hit a wall - so far I have only got 'source at least three suppliers who can provide the service/item' (terrible I know). I was wondering whether anyone out there has any further best practices that I can add to this for my teams who will be self-serving any requirements? Thoughts and comments will be gladly received!