Never got one above 20 in Origins that I can recall, but you can get that achievement from Awakening, anyway. If you're very concerned about it, I'd invest in the Archivist's Sash before finding out about anything, and then go do a lot of research. The boost in experience from that item can really add up, although I don't know how many levels you could possibly gain from codex entries.
As far as items, the thing you want to do is unequip everything you actually want to keep and have it in your inventory. Anything on a companion will be ignored. You can do this after going through the end of Origins by loading an "epilogue" save, so you don't need to beat the Archdemon with a naked party or anything.
It helps to sell anything extra, your inventory will fill up fast in Awakenings if you try to carry too much, anyway.