My 2 cents after doing this long enough to recognise it

- Aim for 3NF but not religiously. Still, if you need a flat table try a view.

- Any ternary relationship can be modeled as a pair of binary relations (you'll never regret keeping it simpler)

- You don't need EAV (Magento is a good example of why you shouldn't)

- On the other hand don't serialize data (looking at you WordPress)

- XML and JSON data types though are perfectly fine when you need to store an object

- Every table should have a primary key (preferably an integer)

- If you really want a string for your primary key make it a candidate key (why, because someone will insist on changing it)

- E/R diagrams are your friend

- So are Venn diagrams for visualizing a complex select