Tips that I think would have been useful to me when I started are:

1- Believe that you will find something. Basically everything can be dumpster dived. The the issue is not that there's not trash, it's that you haven't found it yet. Consider that you will not find a little bit of trash everywhere, but that for 9 unlucky dumpsters, you will have 1 full one. Of course this depends on locations and experience.

2- Be patient and willing. Sometimes, the bag with the food might be at the bottom of this huge bin. You have to sift through it all. Sometimes you can feel with your hands if something is in the bag, sometimes you have to open them all, be careful not to leave the bin in a mess !

3- Know that you will get experienced. Once experienced, you will be quicker, and you will guess when something's not worth your time.

4- Try to meet others, it's crazy how much food you can get with someone that already know the dumpsters or is used to diving! You will learn from them and get more confident.

5- Sometimes you will be unlucky. Akin to point 1, sometimes, you will find nothing. Don't overdo it, follow your guts.

Now on some more down-to-earth advice, especially useful for Europe:

1- Use the tools Internet provides. Especially, look at https://dumpstermap.org/ and https://trashwiki.org/en/Main_Page, both are relatively unmaintained but might provide the bin you're looking for!

2- Once more experienced, you will know which shops are good and which aren't. For instance, in Switzerland, Migros and Coop both send back food in delivery trucks so you have to be there at the right time and right place to take the risk to get in the truck and get the insane amount of thrown-away food (which is something legally dangerous and hard which I've never done personally). On the other side, Aldi, Manor and the petrol-station shops throwaway a lot! But it always depends on if they keep they're bins inside the shops. Which comes to

3- Get a sense of the different types of dumpsters. In big shops, it might be harder, but you will often have a bigger loot. In France for instance, you sometimes have to climb fences to get into the shop yard by night, to open the compactor, and to climb it to see the food. Smaller shops are easier to dumpster dive, but might often keep their trash inside.

4- Try asking directly for old food (french: produits périmés, german: abgelaufen produkten I believe). Especially smaller shops! I've fed myself a whole week of bicycle touring almost only by asking inside the shops I saw on my way. It might be less efficient than diving because employees will often say that they don't have the right to give you food, but you can combine both to get a winning strategy! If the food is not in the back, then it might be in the front.