Easy peasy, all u need is a good weapon (for fast killing) + head with +19-25% EXP gem.
How to:
>> 1. Start Act 1 Quest 6 (The Broken Blade --> Retrieve The Beacons)
>> 2. Find entrace to Crypt Of The Ancients
>> 3. Kill all monsters in the cave
>> 4. Leave game. Resume. Repeat.
Tips:
>> Work on all difficulties - of course most efective on Hell.
>> Once u enter Crypts Of The Ancients, u'll spawn in cave every time u resume game.
>> In most runs there is +25% EXP chapel - start with quick run through to get buff.
>> Kill those Returned Summoners at the end (those guys that spawn skeletons) = more exp.
>> Good way to get some extra gold (with +GF gear).
Quick video:
From all these solo leveling methods I tried so far, this one work best for me. It's not that boring like the other ones.