Once considered the cockroaches of the sea, these invertebrates had it easy for quite awhile until we figured out their delicious secret. Even more fortuitous, the spiny variety found in California and Baja are sweeter than their clawed cousins. Currently the US lobster market is worth estimated $23 Billion, with around 95% exported to China. With such high overseas demand, this makes them unlikely to be found in local supermarkets. If only you could walk into the surf and find them.
Just a Few Important Ground Rules: [2&3]
1. Season opener for California Spiny Lobster runs from the Saturday before the first Wednesday in October until the first Wednesday after March 15. 
2. Lobster fishers/divers may not land more than seven California spiny lobsters on any given day, and may not have more than seven in their possession at any time.
3. Fishers/divers must carry a lobster gauge, and any short lobster (less than 3 1/4 inches) must be returned to the sea immediately. The minimum size is a carapace length of 3 1⁄4 inches, measured along the midline from the rear of the eye socket between the horns, to the end of the carapace. 
4. A report card for the season must be bought, filled in and returned before April 30 after the season ends.
5. Commercial and recreational traps must not be interfered with.
With that out of the way, lets get down to how to find these guys. Lobsters are nocturnal meaning they hide in caves and crevices during the day, coming out only at night to scavenge and hunt for mollusks (sea urchins, mussels, clams, worms, and snails). This means that you are much more likely to find them free-swimming (walking on the ground). A bug out in the open is much easier to grab but that doesn't mean you can't get them in the day hiding in their holes.
Lobsters like a particular kind of structure, meaning they wont be found in all kinds of caves. They generally don't like caves and crevices that have very large openings or where there is a lot of current. Rock piles are excellent places to check as they usually offer many spaces and escape holes. I like to think of the ideal rock structure as a large "table" rock on top of smaller rock "legs". Break-wall are another great area for this same reason, but be very careful diving break-walls. There are typically many chambers deep, and its easy to venture in too far lured by many bugs, then get disorientated, blinded by silt, and trapped. This is very dangerous for scuba and deadly for free-divers.
There are a few grabbing techniques you can use depending on the situation. The most effective in my opinion is the Pinning Technique. Moving quickly, as soon as you see them, use your (gloved) hand to dart out and pin the lobster to the ground or cave wall, then when secured, get a better grip on the carapace or knuckle (base of the antenna). Keep in mind, they will scuttle backwards quickly. Its good to lead them a bit, aiming a little behind where they are. The second method is the Knuckle Technique. This is used when pinning won't work because they are in a small hole/crevice with just their front exposed. Its very easy to break the antenna if you're unable to get a hold of the knuckle (thick base of the antenna). They will dart back quickly with a fast flip of their tail breaking their antenna so you have to be precise.
Armed with this knowledge, you should be much better prepared to fill the freezer (I recommend freezing lobsters whole). Make sure you keep them alive on the car ride home too, dead bugs spoil very quickly. Its important to keep their gills wet/moist with salt water (freshwater will kill them). This can be done by throwing some fresh kelp (wet towel or wetsuit) in the bag with them. Also don't get discouraged of you're not seeing them or you keep missing the grab, I spent about two years missing bugs before I started getting the hang of catching and spotting.
Reefs are a good place to start, finding the right kind of structure is key.
Moray eels and lobsters like to hang out together so be careful where you place your hands. Look out for sea urchins too!
Always follow rule 5, but know these commercial traps are purposely placed near good reef and structure. Also at night, the bait in the trap will draw in bugs around the trap.
Always be respectful and follow the law.
Float lines are a good way to mark caves and avoid loosing spearguns.
Small lights are great for checking caves.
Retractable lanyards are amazing for lobster gauges and lights.
Try to get proficient at sizing bugs by looking at them. If it looks short, just pass on them. Less time trying to catch short bugs means more time hunting legal bugs.
If you're looking for how to tail and prepare your catch, San-Diego Steve  has a great article on it.
Also if you're looking for a way to upgrade your reef gun, take a look at the Kimera SideSlip . It combines the durability of a flopper shaft with the holding power of a SlipTip. Can be used on any shaft.