For a tomato based sauce, far more puree than is normally suggested, time - for the onions to soften and change colour before adding anything else, for the spices to cook and then infuse throughout the cooking process, for the liquid to reduce - and then further attention in the last five minutes for any last tweaks to the flavouring.
For anything savoury, sea salt flakes rather than standard salt. Means a lower total sodium with far more flavour than adding table salt (which I think tastes disgusting).
For anything containing fresh herbs, more than is conventionally used or if it's homegrown, less but they're normally far stronger flavoured, put in when it's best for the flavour.
For spices, well, coriander in particular, homegrown that has run to seed rather than brown pellets in a jar that may have been harvested four years ago. The taste is completely different.
Tamarind paste for a sweet/sour/darker savoury flavour.
Where a recipe would be frequently expected to have cream or other dairy in it, a cooked potato blended in or some potato/cornflour/arrowroot slaked in water/stock does wonders for the feeling of richness and texture without the expensive/forgotten/nasty allergen.
For stews, curries, etc, it's not even remotely cooked until it makes a sound like a volcanic mud pool and any oil or fat in the ingredients has risen to the surface.
Allowing meat and fish to come up to room temperature whilst fully covered before cooking. Far less chance of cold or underdone spots that way.
Pick a cooking method and ingredients that enhance the flavour, such as using avocado oil for something including bacon or rice bran oil rather than olive, steaming rather than boiling potatoes so they don't get soggy or grainy.
Other than that, I use things like Gochjang, Miso and suchlike frequently for extra depth in flavour. They also keep for far longer than the tubs suggest.
Got another couple, but I'm keeping them absolutely secret...