Free Flower Pikmin
This tactic is only available at the beginning of the game, so if you've passed the start and don't intend on restarting, there's no point in reading any further.
The first level, on the first day, is the only time when you are not being timed to complete your mission. If you leave Pikmin in the ground once they come out of the Onion, over time, they will form a bud, and then will flower. Since you have unlimited time on this first stage you might as well wait for them to fully flower before plucking any of the Pikmin out of the ground. This works on all of them except the first red Pikmin that comes out. That one will never flower, as you need to pluck it to continue the game.
Pikmin that flower are not only faster, but are also stronger in combat, so the more of them you can get the better.