HP Washing

What is HP Washing?
HP washing is a method of obtaining more Max HP for your charcter.

Why should I HP wash?
If you want to have more HP in order to tank hits without dying from certain bosses.

Okay, how do I HP wash?
Ah, there's the rub. That's why I'm making this guide. If you just want to go out and do it, skip to "Tell me what to do".

HP washing in general is very simple. The basic idea is this:
1) Put an AP into HP (note: level up AP gives more than a reset AP!)
2) Use an AP reset to take a point out of MP, and put the point into main stats.

So, in order to HP wash you need to have extra MP to take the points out of. There is a minimum MP for each class, so the game will eventually stop you. The main concern with HP washing is getting more MP so that you can wash more points into your HP.

Every time you level up, you gain: 15 + (INT / 10) +/- 1 MP. Ergo, each time you level up, you will get an extra +1 MP for each 10 INT you have. (Additionally, you get a bit more than the minimum MP naturally anyway) What a lot of people don't know is that if you have extra INT, you will also gain more MP whenever you put an AP point into your MP pool. Knowing this, it is actually possible to wash infinitely.

For the data on minimum MP and reset data (I'll integrate this myself later): http://www.basilmarket.com/MapleStory-Guide-HP-Washing-A-Comprehensive-Guide-329.html

Well, how much INT do I need?
That depends on what class you are and how much HP you want. Remember that if you ever don't have enough HP and want more, all you have to do is get enough INT that you gain more MP per AP than you lose when taking an AP out of MP. However, here's my recommendations. These are just guesstimates, so they are not accurate.

Magicians: Irrelevant due to Magic Guard.
Shadowers: Irrelevant due to Meso Guard.
Heroes, Paladins, and Buccaneers: The only reason you would want to wash is to do Bigfoot without HB. You won't need more than 10 base INT, and you'll probably need to wash between 10 and 30 times.
Dark Knights: You want to wash to 18.6k HP base (30k with HB) to use berserk easiest. You will need around 10 INT and to wash around 120 times, more if you want to get it earlier.
Night Lords, Bowmasters and Marksmen: It really depends on your HP goal, but for simply doing Horntail runs, my recommendation would be 50 INT. You will probably need to wash 60 or 70 times. To get to 12k base HP, my recommendation would be 120 INT. You will probably need to wash around 300 times.

Okay, that's all nice, but tell me what to do!
You got it.

There are two options: Put some of your AP into INT, and/or equip gear that gives you additional INT. Most players choose to do a mixture of both. The more HP you want, the more you will want to invest in INT. For information on what gear to use to gain INT, see my Gear Guide.

Before you level up, be sure to equip all your gear that provides INT. After you level up, put 1 AP into HP. Use an AP reset to take out of MP, and put in main stats. Repeat as many times as you can.

Hey, I still need more HP but I can't take any more out of MP!
Don't worry, you can still fix it. It's not easy, but here's how.

1) Equip INT gear. How much you need depends on your class. You need to gain more MP per AP than you lose from taking out.
2) Use an AP reset. Reset out of your main stat and put a point in MP.
2a) If you need more MP, use another AP reset. Rest out of MP and put into main stats. Repeat steps 2+2a as needed.
3) Use an AP reset. Reset out of MP and put a point in HP.
4) Use an AP reset. Reset out of MP and put a point in your main stats.

TL;DR
http://www.basilmarket.com/MapleStory-Guide-HP-Washing-A-Comprehensive-Guide-329.html