I know this has been mentioned before but this is my proposal. California should be divided into 4 states, 2 for counties south of the "line" and 2 for counties north of it. I will call them by the name of their largest city.
1. Los Angeles aka "West California" (Los Angeles County, Ventura, Santa Barbara, San Luis Obispo, Kern).
2. San Diego aka "South California" (San Diego County, Orange County, Riverside, San Bernadino, Imperial).
3. San Francisco aka "San Francisco Bay" (San Francisco County, Monterrey, San Benito, Santa Cruz, Santa Clara, San Mateo, Alameda, Contra Costa, Marin).
4. Sacramento aka "Central California" (Sacramento County, Napa, Solano, Sonoma, Yolo, San Joaquin, Stanislaus, Merced, Fresno, Kings, and all the remaining counties).
The boundaries aren't fixed and of course the people in border counties could vote on which state they would want to join (e.g. Kern County, Sonoma County).
This is very similar to the earlier proposal to split California into 6 states. The main difference is that my state of Sacramento would be split into 3 states, with Central California (including Kern county), North California (Sonoma, Solano, Napa, Sacramento, and west), and the state of Jefferson.
This would give the states more political power, with 8 Senators total vs 2 Senators now.
See also http://www.nationalreview.com/article/444384/california-secession-bad-idea-division-two-states-better-idea