Lista della spesa
This page contains links to chapter-specific lists of items you should try to have in your inventory to complete all the quests in the game, whether or not you can get esperienza by doing them.
At the beginning of each act, before you do anything else, have a quick "whip-round" the area. Enter every single building you can enter and open every single container. Take as much stuff as you can carry and sell whatever you don't need (check the lists below for the stuff you need). While you are loaded up with potential gift items, try talking to as many NPCs as you can find, but always begin with the most minor character. Many of them will give you information in exchange for gifts or money. This helps shed inventory as well.
If you begin speaking to the main characters too soon, you may advance the plot prematurely and miss out on an opportunity to get something for free, or just get "some". It is always a good idea to keep a close eye on your journal entries. They really are useful and often tell you what to do next, particularly if you read each phase entry for each quest as you get them.