Note: The Bound Sword spell is recommended when trying this since you are able to conjure two of these at the same time and thus gain experience faster. This works best when there is an enemy attempting to fight you, but you are at an area that they cannot reach you (for example, fighting a troll or bear while you are too high for them to actually hit you). Use the Bound Sword spell, put the sword(s) away, then repeat the process. Doing this will increase your conjuration levels very quickly.
For this trick you must have dragons roaming around and being far enough in the Dark Brotherhood to have Shadowmere. When you find the location of a dragon, Fast Travel there so Shadowmere can be with you. Start the battle with the dragon. Do not worry when the dragon goes after Shadowmere, as Shadowmere is invincible and will fight and eventually will kill the dragon for you. Use the Bound Swords spell as described above and it will have the same outcome.