Often, an employee is subjected to unlawful discrimination and retaliation while still employed and weeks or even months before being terminated. Although very "tempting," suing an employer while still employed is problematic. First, you are very likely to lose a job (just because it's unlawful to terminate an employee who exercises his legal rights to bring legal actions, doesn't mean that the employer simply can't violate the law and do it). Secondly, your recovery in any case is likely to be insignificant because you have not sustained any loss of wages, which is at the core of any settlement or judgment.

Despite the above, there are a few important things you can do to line up your "weapons" if and when you get unlawfully terminated as a result of discrimination or retaliation: 

1. Make sure that your boss cannot blame terminating or demoting you on your performance. This is a critical time to do the best you can, as the employer's love to use poor performance as excuse for termination, because it's so subjective and relatively hard, although possible to argue against.   
 
2. If there are any witnesses to unfair treatment, get their statements in writing if possible, and keep their contact information. In many cases with opposing sides having opposing story, witness statements are crucial.   
 
3. Keep all relevant documents, emails and other documentation proving discrimination, retaliation, or your complaints to HR or your superiors about the same, as they will be of critical important when proving your case in court.   
 
4. If you witness discrimination or harassment against yourself or others, complain to HR in a courteous but firm manner and ask for investigation in writing.   
 
5. If you are terminated, do not sign any releases in exchange for severance before you consult an attorney, as signing a release usually extinguishes all legal claims, committing the signing employee to never sue the employer for any violation.
 
Then, when the time comes and you are well equipped to fight, the results will be better and the process will be likely shorter, as the employer, facing substantial evidence against them will likely want to settle faster. 

California Labor Code section 98.6 prohibits employer from terminating, threatening to terminate, demoting, suspending, or otherwise discriminating against an employee who participates as a claimant or as a witness in proceedings before the state Labor Commissioner or otherwise exercises his or her rights under the Labor Code.

Labor Commission proceedings include claims for unpaid wages, claims for retaliation against employees for complaining about violation of wage and hour laws (such as meal/rest breaks periods, overtime, etc.), claims against en employer for preventing an employee from engaging in political activity or running for office or for requiring an employee to participate in political activity. 

The employee has six months after the act of discrimination occurs to file a complaint with the Division of Labor Standards Enforcement (DLSE) as per Labor Code 98.7(a). Only employee who make actual complaint or claim are protected. Labor Code 98.6(b).  Such an employee is entitled to reinstatement and reimbursement for lost wages and work benefits caused by the unlawful acts of the employer.  

This protection is especially important to those employees who make a wage claim or overtime claim against their existing employer by filing a wage claim with the DLSE (Department of Labor Standards Enforcement), and who are concerned about being retaliated or fired as a result.