As you travel through the world, you can grab all sorts of things on your way to the next location. Your backpack is basically a "endless" storage (30000), so if you have the time, try to grab everything you can get. Almost every item can be used or converted into other items by crafting, cooking or processing at a NPC.
Every defeated enemy can drop items, from artifacts, crafting materials, character evolution items, and etc. Always try to pick up everything. You can craft or trade low level items at the alchemist to a higher grade replacement.
Ores, crystals and non-direct lootable flowers like dandelions or burning flowers, which you have to actively strike with a elemental skill to loot are very valuable. If you think you found a good spot, try to mark them on your map, it will become handy later. Note that some flowers need more than 24h to respawn.
The same goes for consumables like apples, meat, all sorts of vegetables, or cooking ingredients. One of the most useful items are healing, regenerating and buff food, which require your exotic cooking skills. By collecting as much as you can, you will never run out of food!
General Farming Tips:
Hit ores with a claymore/two-hand wielder - they will destroy them a lot faster
Some flowers need a special elemental active before they can be picked up
Use large aoe spells to farm meat (chicken or boars)
Always check your bag and try not to waste crafting time for special materials
Set map markers on locations with alot of materials to come back regularly (or use this map)
Remember that you can loot materials in any other player's worlds. If you are missing out on some materials, try and ask a buddy of yours if you can drop by ;)