Even in winter, it is possible to wear beach attire most days. However, after the sun goes down, you will most likely need something warmer to wear.
Also, if you intend to join the locals at any of the night spots, you might consider dressing up a bit as this seems to be an up-scale beach town and some places seem to have a dress code.
Luggage and bags:
If you're planning to play on the beach a lot, I would suggest you bring a bag you won't mind getting sandy. I would also suggest that the bag be portable, like a sports duffel bag or a small backpack.
Clothing/Shoes/Weather Gear: During spring, summer, and fall the days can get pretty warm, 70 to mid 80s farenheit. Since you'll be right on the coast, the evenings can get kind of cool. So tank-tops, t-shirts, and shorts for during the day. Pants and sweatshirt for the evenings. I also suggest sandals or slippers for easy removal of shoes.
If you plan to do some fine dining in the evenings, most places are casual so just slacks and Polo T's for the guys and something comparable for the gals.
As for the winters, it's been unsually cold the last couple of seasons, 60s to 70s during the day and dipping down as low as mid 40s at night. So sweater/sweatshirt and thin jacket are good to have.
Camping/Beach/Outdoor Gear: If you plan to be at the beach a lot, beware of the sun. So bring a hat or umbrella to give you shade. Sandals or slippers are a plus.
Bring sunscreen too!
The weather is great all year round so definitely bring shorts in the spring and summer time. Bring a light jacket for the autumn months.
Camping/Beach/Outdoor Gear: There is no camping allowed here, but just north of Manhattan Beach, is El Segundo. They have a little motor home beach camping area.
Miscellaneous: Bring your surfboard, if you surf!!!