The first race of the season is on the first Friday after Daylight Savings Time begins in April, and the races are held every Friday right through the end of September.
Summer evenings in Berkeley are usually cold and windy. This increases the demand for crew, because most boats sail faster with more "live ballast" (sometimes known as "rail meat") to help keep the boat from heeling over too far. That's why there's an entry-level crew position called "sandbag."
As you get more experience you'll be asked to do more and more sophisticated things, starting with tailing sheets and grinding (cranking) on winches to trim sails, and moving up to spinnaker rigging and trimming. After about half a season of regular Friday night crewing you will probably be in demand on mid-level race boats, and have lots of opportunities to race on boats in more serious weekend events. It generally takes years of experience, however, to become valuable to the boats that regularly win these weekend events.
To get a crew spot on a boat for a Friday night race, show up at Berkeley Yacht Club at or before 6:00 PM. Most boats don't actually leave their docks until 6:10 or even 6:20, but if the boat is berthed in another part of the marina and the skipper is at the yacht club looking for crew, some extra time is required.
Crew usually wait at the top of the O-dock gangway, not far from the yacht club front door. It's appropriate to ask "need crew?" to anyone who walks by looking like they might be a skipper. This is a little like being eyed over in a singles bar - but this is a seller's market, and the selection criteria are very different. The skippers are looking for sailing ability and experience, and sometimes muscles and body mass, if it's a windy evening.
If the skipper answers your "need crew?" with "maybe, but first I have to see if some people I invited are going to show up" that might be code for "yes, but I want to see if I can find someone better first."
You have no choice but to cooperate, if you're new. Skippers have every right to look for people they know before taking on a stranger. Be patient, the same skipper might be back in 10 minutes with much lower standards!
You can improve your chances dramatically by bringing a small consumable treat. Designer chocolates, up-scale cookies or muffins, or other snacks. On Twilight Zone, the yellow Merit 25 berthed just inside the O-dock gate, no-one has ever been turned away if they arrived with a nice tray of fresh sushi for the whole crew.
Another tip: Have all your gear with you and be ready to jump aboard. Don't say "I have to go back to the car to get my gear" or "I have to make a phone call first" after the skipper says you're on. They're probably short for time and want to get going right away.
On the water, expect it to be cold and wet. Sailing on an unfamiliar boat with an unknown skipper and crew, you should definitely wear a life jacket - you don't know how long it might take them to pick you up if you should fall overboard. If you don't have your own life jacket (often called "PFD" for "Personal Flotation Device,") ask the skipper if they have an extra one for you, before you get on the boat. But note that some boats do not carry extra life jackets that are comfortable to wear while racing (which is not a good sign). Comfortable life jackets are cheap enough, and you should own your own if you're at all serious about this. If you are a non-swimmer or a weak swimmer, make absolutely certain that the skipper is aware of this, so that they apply the safety standards that they consider appropriate (which might be stricter than your own - and probably for good reason). Always wear a life jacket if you're a non-swimmer. Also, in a situation where two or more people fall in the water, it's important that the skipper knows who to rescue first.
You should have some form of water-repellant pants, aka "foul weather gear" to keep your bottom dry. The style with the bib front and suspenders are universally prefered. For the top, a sweater and a light waterproof shell is usually sufficient - you really don't need the expensive top to the foul weather gear set, especially since they can be very bulky when worn along with a life jacket. A wool hat is also nice to have along. If you don't have any of this gear you should still try for a crew spot, but you might have to be more selective (avoiding the smaller/wetter boats) or might have to mooch loaners off other crew or skippers. Bring more cookies!
Note that even strong swimmers often have trouble just keeping their heads above water with foulies and boots on. There's no way to anticipate how difficult this is 'till you try it - so take the recommendations for life jacket use seriously - even if you're on the swim team.
As you get more serious about crewing, you'll buy sailing boots and gloves, and you should also have a watch with a count-down timer. You'll also have a tide book (free at most marine supply stores), a copy of the race instructions, and you'll memorize the signal flags used to signal the race courses.
Sailboat racing can be dangerous, and minor injuries are not unusual. You can be hit (very hard!) by the boom, you can (and probably will) be cut, scraped, banged, and bruised. You can have tendons pulled and bones broken if you don't understand the forces acting on various parts of the sails and rigging, or if you attempt to do things that are way beyond your experience and ability. So be careful, and always ask first if there are any safety concerns. Don't fall overboard, keep your head low, and assume that any piece of heavily-loaded gear could break at any instant.
Remember that even if you seem to be trading food for passage, or even if you seem to be getting food or drinks in return for your services as crew, it is usually understood that you are neither a paying passenger nor a payed crew. Any exchange of valuable consideration should be voluntary. Most skippers and crew consider themselves to be co-participants in a competitive sporting event, and crew voluntarily assume the risks that are associated with this participation. A few skippers (usually the more diligent and safer ones) will ask you to sign a "guest register" that includes a statement to that effect, or a liability waiver of some sort. These "waivers" of course will not remove the normal obligations of the skipper to operate the vessel in a safe and non-negligent manner - so you should feel free to sign such agreements without fear of signing away any important rights.
After the race you will be expected to help put the boat away. Even if you have no idea how to help, it's poor form to run back to the parking lot or yacht club until the skipper says it's okay to leave. There's usually a barbecue and trophy presentation at the yacht club, and it's normal etiquette for the skipper to buy one round of drinks for the crew after the race. But this is hardly universal practice, so don't misinterpret if it doesn't happen.
The post-race postmortems and general-purpose schmoozing at the bar are often the best parts of Friday night racing. Even if you don't get a ride, or don't get to the club in time for the start, prospective crew should feel free to stop by and check it out.