Adament:Use in Battle, Include in Party (both work equally)
Bashful:Cologne Massage
Bold:Include in Party
Brave:Use in Battle
Calm:Use in Battle, Include in Party, Cologne Massage
Careful: Daycare, Cologne Massage
Docile:Call from Hyper Mode
Gentle: Daycare
Hardy:Use in Battle, Include in Party
Hasty:Use in Battle
Impish:Use in Battle
Jolly:Use in Battle, Cologne Massage
Lax:Include in Party, Cologne Massage
Lonely:Cologne Massage
Mild:Cologne Massage
Modest: Daycare
Naive:Include in Party
Naughty:Include in Party
Quiet:All methods work equally
Quirky:Use in Battle, Daycare
Rash:Cologne Massage
Relaxed: Daycare
Sassy:Use in Battle
Serious:Call from Hyper Mode
Timid: Daycare
BTW, when I say "Use in Battle", that means when you send it out. And "Include in Party" means walking around with it without battling. So for your Lax natured Pokemon, walking around and using the Cologne Massage are the best natures.
Oh, and this is for Colosseum. If you're playing XD, it might be different.