Minecraft Forums

1. First, go into your world and add any potion effect you want to you. Slowness, Fire Resistance, anything you might want. It doesn't matter, since we'll be changing that anyways. For this example, I'll be using Slowness.
2. Next, close Minecraft and open up NBTedit. You must do this as you risk corrupting your world otherwise. Open up the level.dat of the world (not the level.dat_old).
3. Go to: Data > Player > ActiveEffects > Entries
4. Now, you see your active effect. Again, in my case, Slowness, which has an ID of 2. Now, you can see 3 things that can be edited: The Amplifier, The ID, and The Duration. The Duration dictates how long the effect will last - with 1200 equaling 1 minute exactly. The effect starts again as soon as you enter the game, so keep that in mind. Using 1200 for 1 minute, you can easily calculate the number you need to get to get your ideal duration. Now, the Amplifier dictates the potency of whatever you have applied to you at the time. Now, this works like this: an Amplifier of 0 = I, 1 = II, 2 = III, 3 = IV, 4 = V, and so on and so forth. Using that formula, you can quickly find out what you need to set your amplifier to, to get your desired potency. And as for your ID, your ID dictates what enchantment you will have. You can find a list of all of the current effect IDs here.
5. Edit your Duration, Amplifier, and ID accordingly. Save before you exit. You'll be prompted with a save option should you try to close before saving, just in case.
6. As soon as you enter your game again, your choice of effect will be active.

Tips:
Invisibility and Night Vision don't do anything.
Weakness, even I, prevents you from punching any mobs. No knockback or damage. (Without a weapon)
Setting Jump Boost very low, even negative, prevents jumping altogether.
Mining Fatigue V and up prevents all mining, even with tools.

A +1 is appreciated if you found this helpful. :3