Spell for carrying more weight in Skyrim

Yes, there will be Nirnroots in Skyrim:

This image is from the G4TV Skyrim demo. You can also see Nirnroot in the Skyrim Trailer.
You can find information about its alchemical properties, location, quests and lore on the Elder Scrolls Wiki Nirnroot page.

Well, lacking a precise formula (even though there certainly is one, the problem can be nailed down mathematically since the only variables are the weight of items, the value of items, and the total amount that you can carry), all I can do is give you some tips:

• If you don't want to invest perk points in armor trees, get light armor. In the end game, light armor will give you the same protection as heavy armor does, but it is... well.. lighter. Source: http://elderscrolls.wikia.com/wiki/Category:Skyrim:_Armor
• Get rid of dragon bones! Although selling them might be a bad idea, since you can craft dragon bone armor if you get deep into the smithing skill tree.
• Stay away from 2H weapons. They are among the most heavy objects in the game, while usually not having a very high value.
• Store your books somewhere! At least I picked up every book that I could find, promising myself to read them one day. But carrying around 50 or so books (there are a lot in Skyrim!) takes up at least a good fifth or your carrying capacity - for little to no value. Get a house and put them in a box!
• Search for gems and soulstones! They weigh almost nothing and are usually decently valuable.
• The same goes for enchanted necklaces, rings, and robes. All of them are ridiculously light compared to their value.

These are the ones that come to mind right now.