Get lots of food. What else is there to say? You have to retrieve one part (the Positron Generator) from one of the Pearly Clamclamps, but that's it.
Here's a video showing how it's done:
Footage of all of the days together is available here.
I start by taking out all of my blues, and twenty to twenty five each of red and yellow. I dismiss these and then go back for more reds, to get the total number of Pikmin up to the magic one hundred level. You should not have to pick any flowers this day (or any day after day three for that matter). I don't take these excess reds with me, as it just makes it harder to walk up the ramp.
I use bomb rocks to get the first Pearly Clamclamp - this saves a lot of time. To do this safely, toss the yellow Pikmin in front of the clam, not in it. They will toss their rocks in, without ever putting themselves at risk. I then let the blue Pikmin take the pearl back - both pearls in fact - because I use a lot of blue Pikmin on the next level.
I use blue Pikmin to knock down the wood wall. This saves a bit of time also.
Other than that I do nothing special on this level. I don't even do a particularly good job of sending the small pellets to the correct Onion.
The next day will use mostly blues, so it is important to have at least 91 flower blues. Be sure to save time for putting all reds and yellows away and walking your blues through a few patches of nectar. Make use of the Iridescent Flint Beetle to get lots of nectar. Having a hundred flower blues is more important than breeding a few extra Pikmin.
The most I've bred on this day is 247 Pikmin. I'm sure it's possible to do a bit better.
On to day 5.