Mail / addresses / vehicle registration
Getting Yourself a US Mailing Address is hassle free:

Mail Center
We have truly thought of everything. If you still receive mail from the US – bills, statements, notices, etc. fear not. There are local services that travel once a week, to Chula Vista and San Ysidro, to pick up your mail. Once picked up, it is sorted and bagged and delivered the next day to your door. With this service you will only get your mail once a week. You will have only one chance per week to send mail out as well, unless you drive it there (the US) yourself, OR have another person drive it there for you.
If you choose to pick up your mail in town, there are reliable post offices in Ensenada, that caters to Americans. Everyone I know who uses this service is extremely happy with it, no complaints.
There is another option and that is a local mail co-op. What this service does is rotate US driving duties to each member. Each member must make the trip up to SanYsidro approximately once per every 6-8 weeks, depending on how many members are joined at the time. After you pick up all the members mail, there is a drop box centrally located where you will place all the mail and members pick up at their convenience. This service is much LESS expensive.
All these services include packages, incoming and outgoing. In most cases you must purchase your own private box, but you can share the box with other people to save money. This gives you a California, US mailing address. Some people I know use a relatives address in another state. They register their cars this way.
The mail services are operated out of businesses like Mr Mail and the like; Mr. Mail, Mailbox Etc. in San Ysidro will hold your packages till your service gets there to pick it up. You can also just do your own mail privately, and drive up to get it yourself. Many people do that as they do not receive much mail.
Receiving mail in Baja is no big deal and never a hassle. There is always some one going to the States that will deliver your mail in an emergency or you need overnight delivery. There is also a DHS pickup service in Ensenada and there is a Western Union Money telegram office if you need money wired. Most of the services offer US postal stamps, envelopes, etc.
In some of the larger touristy towns you will find an Internet Business Center, where you will be able to get a mailbox and send/receive US mail, use the internet, photocopies, scanning, private work cubicles, faxing, and phone to US and Canada all in once place. San Felipe has one such place.
YOUR PHYSICAL MAILING ADDRESS
Most people want to have a US address. This facilitates getting your social security checks, bills, retirement pay and the like. Again this is no problem. With your mail service I described above, you will get a US post office box. Absolutely not a problem what so ever. My house doesnt even have a physical address and i dont live on an officially named street. Its almost like i am invisible. If you like your privacy, it couldnt be more perfect.
There are ways to get around registering your car in California too. Personally we use South Dakota. You dont have to show proof of auto insurance. You don’t have to live there. You dont have to get a Emissions/SMOG test like in California. You can do it all on the internet. They mail you your plates in the mail and its very inexpensive. You get a new SD title and registration. Many people here dont have US car insurance because they dont drive there much, if ever. That makes getting a car registered in the states a little tricky, for the times you must go to the States, but with the state of SD this is not a problem. Most full time RVers you see on the roads in the US have SD plates. They just are interested in the revenue it brings to the state.
Many people do have to go up to the states for doctor appointments and need insurance. So, for your convenience, you can purchase US auto insurance by the day in Ensenada. Seriously, everything is taken care of here in Baja. Have an open mind and you will enjoy this wild ride you are on.