Focus: What's going on in your head
More than technical operating skills, splitting lanes requires the ability to be present, evaluate and make advance decisions about your course of travel while maintaining the ability to react quickly to unexpected events.
More than anything else, you need to use your eyes to take in as much as you can about your environment. This means seeing more than just what your eyes are focused on. When I'm blasting through traffic on the 405, I'm not picking out things to focus on or look at, but instead keeping my eyes up and forward with a focal point somewhere roughly an 1/8 mile ahead of me. If you have targets, you run the risk to fixate on them, block everything else out and set a collision course. Don't do that. Keep your field of view wide and avoid focusing on one specific thing. The immediate foreground isn't in focus, but I still give it awareness. Learn to use the out of focus corners of your vision and if a car grabs your attention, slow down and make sure it's safe to pass them. At first, this will be extremely hard and will limit your speed. If people surprise you and you feel an adrenaline rush, that's bad. Slow down until you can see where you're going and where you are. When you're first starting out, it will be mentally draining to pay so much attention to so many different things. Take it easy and you'll get better. Once you can see all the cars, start paying attention to the negative space between them. Search for narrow spots and prepare for them in advance.
In addition to mentally calculating your position in relation to others, you must also be able to evaluate traffic to spot untrustworthy drivers. It's like a Rorschach test you don't want to fail. Look in the drivers mirrors and back windows. Are they talking on the phone, watching a movie, eating/shaving/brushing their teeth, screaming at their kids, etc? These people are what I would call untrustworthy. You can't depend on them to stay in the center of their lane, use their turn signals or look before they make snap lane changes.
Be extremely judgmental toward other drivers. If there's ever a time to stop being politically correct, it's when you're sandwiched between lanes on the 10 freeway. Start profiling. Is that lady driving an Escalade on 24" rims while texting on her rhinestone encrusted Blackberry? Does she have a "Children are a gift from God sticker" on her back bumper? When I see this lady, I give her a wide berth. How about the guy in the '89 Civic with a double-decker wing, coffee can exhaust, seat leaned WAY back and broken driver side mirror? How about the old, vaguely middle eastern man in the beat to shit minivan? Would you trust these people with your life? If a person gives me any reason to think that they might be aggressive, absent minded, stupid or is otherwise suspicious, I give them my full attention. People in unfamiliar places (I'm looking at you 113 year old Chinese lady with Nevada plates in the Benz) tend to dart across 4 lanes of traffic to make that off-ramp they weren't expecting. People that have a lot of bumper stickers tend to do make bad decisions. Riders of Goldwings and Harleys will often attempt to lane-split, holding you up, and rarely check their mirrors. Don't even get me started on Prius drivers. When you come across anyone that doesn't immediately come off as a competent and trustworthy driver, slow down and wait for them to make whatever bad move it is they're going to make. If it seems like it's going to be awhile, go around or wait for them to stop and proceed cautiously.
There are no hard and fast rules on who you can trust, but I've found that most guys wearing flat-billed hats in lifted trucks are actually very good drivers. They pay extra attention to motorcyclists, move over for you and often wave. If drivers wave at you, wave back. Keep them happy and they'll keep being nice. Professionals on their way to work, often driving boring commuter cars, are generally trustworthy. Other motorcyclists wearing complete gear on dirty bikes are usually safe. If you commute on your motorcycle, you'll notice that you end up seeing the same people every day. You'll cross paths with the same motorcyclists on opposite sides of the freeway, and see many of the same car drivers. Knowing the roads and freeways you ride help quite a bit as well.
Watch for patterns
You need to develop a sixth sense to tell you what cars are going to do before they do it. Don't worry, that's not as paranormal as it sounds. On the highway, is one lane of traffic slowing down while another continues apace? If so, expect drivers to try and dart from the slowing lane into the one where traffic is still flowing. In stopped traffic, has one lane started move before another? Again, expect drivers to shift into lanes with higher speeds, even if its futile.
By lining these cars up for a pass as they're adjacent to each other, this rider is reasonably sure that neither car will attempt to merge lanes.
Riding between lanes of equal speed traffic, watch for gaps to open up that cars could turn into. Avoid sitting next to those gaps. Sometimes, passing two cars while they're next to each other is safer than waiting until one is in front of the other. If a car has no way to shift lanes, then it probably won't.
Take advantage of the safety benefits
While navigating a constantly shifting, unpredictable, deadly obstacle course does have its risks, splitting lanes will help you overcome some of the inherent safety deficiencies a motorcycle is saddled with.
Removing himself from the shortening traffic column by moving between lanes could have helped this rider avoid being rear ended.
Lacking any sort of crumple zone and visual awareness among dozy drivers, we're uniquely exposed to rear end collisions. Instead of sitting in an empty lane at a red light, waiting for a truck to rear end you, pulling in front of a car gives you a free crumple zone and much more visual area and lights to catch the attention of that texting teenage girl approaching from the rear.
There's a lot more to learn
By now you've read almost 2,600 words about lane splitting. The reason I have so much to say is because I've gathered each little bit of my lane splitting advice, near miss stories and observations over 3 years of commuting 80 plus miles per day on four of LA's busiest freeways in all weather conditions. While this lays out a framework and some things to keep in mind to get started, this is a skill that is truly about practice.