How to Increase My Pokemon's Happiness? - Pokemon Diamond

Some nice Pokemon evolutions involve maximizing the Pokemon's happiness. How to do this, you ask? The answer is actually quite simple. Below are a variety of methods to increase your Pokemon's happiness. To check your Pokemon's happiness, go to Poketch App #6 and tap on the Pokemon. If it has two big hearts, then your Pokemon is at its maximum happiness. All you need to do now is gain a level to evolve your Pokemon.

• Having your Pokemon hold the SOOTHE BELL is a great way to help increase happiness. The Soothe Bell doubles the rate at which happiness is gained so you can get your Pokemon to evolve a lot earlier than it would if it weren't holding the bell.
• Carrying your Pokemon in a LUXURY BALL will also increase the rate. These can be bought in Sunyshore City or in the Elite Four castle. However, these can't be used on hatched eggs.
• Give the Pokemon a massage in Veilstone City. The massage lady can be found in the first house to the right of the Veilstone Gym. This can only be done ONCE PER DAY.
• Use your Pokemon in battle and level them up
• Feed your Pokemon VITAMINS (Iron, Calcium, Carbos, Protein, HP Up, Zinc). These can either be found around the region in various routes or be bought in the Veilstone Department Store for $9800 each. Each vitamin has its own special property and will increase your Pokemon's Base Attack (Iron), Defense (Protein), Sp Attack (Calcium), Sp Defense (Zinc), or Speed (Carbos).
• Feed your Pokemon an ENERGY DRINK (Fresh Water, Lemonade, MooMoo Milk, Soda Pop). The Fresh Water, Lemonade, and Soda Pop can be found in vending machines on the top level of the Veilstone Department Store. The Fresh Water costs $250, Soda Pop $300, and the Lemonade $350. The Soda Pop is also a sweet drink, so give it to Pokemon who likes sweet stuff. The MooMoo Milk can be found in a tavern at the end of Solaceon Town. It can be bought for $500. These drinks can only be fed if your Pokemon is low on HP. The Fresh Water heals 50 HP, the Soda Pop 60 HP, the Lemonade 80 HP, and the MooMoo Milk 100 HP.
• Feeding Your Pokemon BERRIES (#21 through #26) can also increase your Pokemon's happiness. However, these come with the price of lowering your Pokemon's Base Stats. Kelpsy Berry = Protein, Qualot Berry = Iron, Hondew Berry = Calcium, Grepa Berry = Zinc, Tamato Berry = Carbos, and Pomeg Berry = HP Up
• Walking or riding your bike around Sinnoh with the Pokemon in your party will also help increase its happiness. 
• Winning Super Contests in Hearthome City. Use Poffins to help increase your chances of winning.
• Feeding your Pokemon POFFINS (If your Pokemon likes Sweet food, then feed it a Poffin with a sweet taste. Check what your Pokemon likes in its Summary)

Another method to check your Pokemon's happiness is in a house at the beach. There lives a man named Dr. Footprint. If your Pokemon really likes you then he'll give your Pokemon a Footprint Ribbon. However, this can't be done for Ghost Pokemon such as Drifloon and Gastly as they don't leave footprints!

Another is to talk with a lady in the Pokemon Fan Club building in Hearthome City (It's the house with a red roof and has a heart symbol above the door) .

Of course, if there's a way to gain happiness, then there's also a way to LOSE happiness! These include:

• Continuously fainting your Pokemon in battle
• Feeding your Pokemon HERBAL REMEDIES. While these will recover your Pokemon's HP, it will lower your Pokemon's happiness because of their bitter taste. They are the Heal Powder, EnergyPowder, Energy Root, and Revival Herb. These can be bought in the Herb Shop at the top of Eterna City.

Pokemon Evolution Guide

Below are the Pokemon that will only evolve when its happiness has been maximized.

BUDEW into ROSELIA (DAY)
RIOLU into LUCARIO (DAY)
PICHU into PIKACHU
CHINGLING into CHIMECHO (NIGHTTIME)
EEVEE into ESPEON (DAY)
EEVEE into UMBREON (NIGHTTIME)
BUNEARY into LOPUNNY
AZURILL into MARILL
CHANSEY into BLISSEY
GOLBAT into CROBAT
IGGLYBUFF into JIGGLYBUFF
CLEFFA into CLEFAIRY
MUNCHLAX into SNORLAX
TOGEPI into TOGETIC