The California seatbelt requirements are codified under Vehicle Code section 27315. Its a lengthy section, but the super relevant section for avoiding fines is subsection (h).
Subsection (h) reads (in pertinent part): "In lieu of the fine and any penalty assessment or court
costs, the court, pursuant to Section 42005, may order that a person convicted of a first offense attend a school for traffic violators or another court-approved program in which the proper use of safety
belts is demonstrated."
Now, no one wants to go to traffic school because that is as expensive as the fine itself, if not moreso. The key to avoiding the fine here is the rest of the last sentence. To do it you need to convince the court to approve a program in which the proper use of safety belts is demonstrated that you HAVE attended, and by that I mean one that you create yourself.
Creating The Program
At the end of the day, the seatbelt law is about saving lives and preventing injuries. Its a noble goal and I will not debate its propriety here. The court, in following the law, simply wants you to wear your seatbelt.
In order to create a program that may satisfy the court, you need to do some research on the internet. Look for any and all references to seatbelt use and why it is important. The California DMV has a whole section of its drivers manual on this. There are many other websites and information, both from the National Transportation Safety Board and other states as well. Be sure to look for information on child safety and passenger safety as well. For this to work it has to be thorough.
Putting It All Together
Once you have located as much information as you can, print it out. The more the better. Sign each page to assure the court that you have read it. Lastly, take a picture of yourself seated in your car wearing your seatbelt. Sign the picture. Put all of this in a small binder. Now you are ready.
Invoking Section (h)
As with every traffic ticket you can go to court or file a plea of Not Guilty by Written Declaration. This approach works well with either. When you argue the case, your primary goal should be a finding of Not Guilty. If the court finds you guilty however, ask the court to accept and approve under section (h) your self-directed study of seatbelt safety. Tell the court that you have learned your lesson, that the proper use of seatbelts has been demonstrated to you in compliance with the law, and that will will obey the law from here on out. If you are sincere and have done your homework the court will probably accept your plea and dismiss the fine entirely.
This is a great approach and has worked for me personally. If you couple this with a plea of Not Guilty by Written Declaration (see my other leagal guide) you can turn what might be a costly and time-consuming trip to traffic court into a modest research and printing assignment for the cost of a few stamps.