I used this method and got 150 levels in about 15 minutes. Here are the steps: (Note, this requires a decent computer)
Step 1: Download TooManyItems mod
Download Link: http://www.minecraftforum.net/topic/140684-123-toomanyitems-in-game-invedit-mar-3-updated-for-modloader/
Ask me if you have any trouble downloading it, I can help.
Step 2: In the TooManyItems items list in your inventory, select the following items and out them in your inventory box:
-Diamond Sword (Shift+Left Click to make the sword unbreakable)
-A stack of Strength II Potions (the ones that make you more powerful)
-A stack of Instant Health Potions
- 5 stacks of 111 (To get 111 in a stack just Shift+Left Click) silverfish spawn eggs (6 stacks will make it lag way too much, 5 is barely manageable)
Step 3: Dig a hole 5 blocks wide, 5 blocks long, and 2 blocks deep in the ground
Step 4: Set the difficulty to Peaceful
Step 5: Set it to be at sunrise in the inventory menu
Step 6: Set the difficulty to Easy
Step 7: Stand at any corner of the hole you just made
Step 8: Go in the settings and set the render distance to tiny, the graphics to fast, clouds off, and Adv. OpenGL On.
Step 9: Equip the spawn eggs and spam (don't hold) the right mouse button as fast as you can (You have to find the right spot on the ground in order to spam really fast, it should be about 2 blocks away from you on the ground in the hole) (I have a laptop so I can rapidly click the right track pad button and the right mouse button at the same time) until there are 555 silverfish in the hole. You should be getting about 3 FPS right now. That is OK.
Step 10: Drink a Strength II potion. Feel the power.
Step 11: Equip the sword and rapidly click the left mouse button until all the silverfish are dead. Make sure to drink health potions when your hunger meter gets low. There should be xp orbs everywhere, blocking your vision. This lasts for about 1-2 minutes.
-You should have leveled up about 30-45 times.
Step 12: Repeat as needed until you're a really high level
Step 13: Enchant ALL the things!