Torchlight if you do not already know about it, is a single player game similar to Diablo 2 that can be purchased for $20 or perhaps less if there is discounts. For those who do, here are some tips on how to manage your inventory inside the torchlight when you're crawling dungeons for shinies and loot from killing tons of mobs.
1) Your Pet
No discussion of torchlight could be complete without mention of the pet system which is actually immensely useful unlike other games. Instead of always having to use a town portal scroll to port back to town, now you can load your pet (a cat or a dog) with junk and send it back to town where it will auto sell everything that is in its separate inventory. To get to the pet inventory, you can click on the icon on the top left icon. To transfer items from your backpack inventory to your pet's, simply open both and shift click the item. Never fear that your items will be lost as your pet will never die. Take care though it will lose its hp quickly if it gets swarmed by monsters and it will flee in terror, in effect prevent you from making use of its abilities for a period of time.
2) Your Stash
You can basically transfer items between your multiple characters through the shared stash. At the very least it doubles as extra inventory space. For those most important items such as legendaries or high level gems, you might want to create a separate checklist or even an inventory spreadsheet with all the stats. For some help you might want to check this sample inventory sheet.
3) Weapon Sets x 2
If you run out of space in your inventory, press 'w' and you'll switch to the backup weapon set, you can then slot two weapons/shields from you backpack into your hands and press 'w' to switch back into your previous weapon setting.
Also if you happen to have multiple gems, if they are the same time you can throw them in for transmutation. Once again its hard to keep track of everything in your stash, you could try downloading or making a sample inventory report to keep things under control and accountable.