EV training. Remember in this example I'm going to go for 252 HP, 252 attack, 4 speed.
1) vitimins. each has 10 EVs and I can use a max of 10 per stat for 100 EVs, no effort. If you have the cash get 10 protein shakes (attack) and 10 HP ups. 200 down, 310 to go.
2) now to find a place to EV train. at the bottom of this guide are some places to go for each stat in this game. remember that every pokemon gives an EV, so dont take it into a random battle unless it gives a stat you want.. on SS I will go to surf at the slowpole well for HP (1 slowpoke = 1HP EV) (avoid the zubats) and surf on route 42 to fight goldeen/seakings (1 attack for goldeen, 2 for seaking).
3) since mudkip is at level 1, I will need a stronger pokemon to do the killing for now. If you give it the exp share it will gain the exp and the EVs.
4) mudkip is at lv 10 and is now strong enough to KO the little slowpoke fools himself. so I move him to the front of the party and change his item. I give him the power weight to hold (check that guide again) and he will now gain +4 HP Evs for every pokemon I kill. so thats now +5 per slowpoke. were in business now. when KOing goldeens I will give it the power bracer, so its now 5 attack EVs per goldeen (6 per seaking). they cut your speed though, so only use them while training.
5) if you get the pokerus, it will double the EVs from whatever you battle. so that means 2 HP evs while using exp share and 10 EVs while holding a power item (or 12 for seakings). So if I am holding the power weight with pokerus I just need to KO 15 slowpokes.