- Sell crapware (nice priced equipment you loot here and there), dismantle relics and junk, keep only "witcher" stuff (crafted, green). By crapware I mean equipment that is not rusty or has a pathetic value under 100 - those low value things just don't loot!
- Except the painting, that thing that gets in your junk goes to Dandelion but is not marked as a quest item for whatever reason, do not sell or dismantle that one.
- Keep a dozen of best weapon/armor tools, the rest of them (means, all of 15% and 40%) just sell or dismantle. Gathering them means tons of weight and you can repair your equipment for cheap just about anywhere.
- Drop all special bolts, those are of no use.
- Alchemical and other ingredients keep max 30 of each, the rest of them feel free to sell.
- For white gull ingredients, don't stack them in inventory but when needed go for inn at hanging tree and buy (innkeeper restockes them just repeat the chatty).
- Don't trash the food, it's lightweighted and useful (assuming you went with what I believe is normal difficulty - the third one called blood and something). Sell however all alcohol! You'll loot tons of food everywhere and you don't really need the stuff to keep you dizzy. Sure there is a quest you need to get drunk, but you don't need any beer or wine in your inventory for that one.
- Sell or drop all books and notes after reading. That includes mysterious recipe (I kept it but noone was interested in it anywhere), Yen's diary and all sorts of written material that feels like it could be needed along the way.
- Books you need for a later quest will be in the quest part of your inventory and you can't sell them (for example Witchers Forge quest where a certain book is needed to open a passage).
You can buy the best saddlebags (+100) in Novigrad by pushing the main story forward very soon (search for Triss).
There is no player's storage nor is such thing needed in this game.
You'll never ever get overburden, you'll never have cash problems to buy diagrams or stuff and will still have everything you ever wanted.
EDIT:
Oh and diablolike socketing stuff? I don't use those at all nor have needed that stuff anywhere. IMO feel free to sell them. If you don't want to risk it, at least sell all those light runestones and keep greater versions.