* Dirt biased tires. Aggressive tires are recommended, such as the Pirelli MT21, the Dunlop D606, TKC 80 or
similar. No street biased tires.
* Ultra-heavy duty 4mm thick innertubes, for those bikes that have tubes, of course. These gnarly tubes are available only from Michelin or Bridgestone. I had a set on my bike that lasted over 5 years and numerous rides in Baja with never a flat. Probably some luck involved, too, but they are no doubt the toughest tube you can get. Alternatively, and even better yet, would be to install mousse inserts. Mousse provides total protection from a flat tire, though they are a bit spendy. They are also pretty difficult to install, and must be well matched to the size of your tires, so if you're not an experienced wrench, you may want to pay someone who has experience and knowledge with these to install them. The Tubliss system is also acceptable, but while they're easy to patch (plug, actually) if you get a thorn in your tire, the inner bladder is susceptible to puncture as well, so if you use these, you are required to also bring a spare standard heavy-duty innertube along.
*A spare front tube (and a tire plug kit for bikes with Tubliss systems installed) as well as tire irons for bikes not running a mousse. A fenderbag made for this purpose is a great way to carry your irons and spare tube on your bike's front fender. Note that you only need a REGULAR heavy-duty tube for a spare, not an "ULTRA" heavy duty one (it would never fit in your fenderbag).
* Toolkit. You'll need what's necessary to remove your front or back wheel, and to tighten most of the fasteners and clamps on your particular bike. For some bikes, the factory toolset that came with the bike is enough to do anything short of an engine tear-down, but for others it doesn't even have what you need to get the wheel off. Check out your needs and put together a small toolset you can carry with you on the bike or in your hydration pack or a fanny pack.
* A small bicycle style hand pump or CO2 inflator and cartridges.
* Long range fuel tank or the equivalent. You'll need to be able to confidently make it 130 miles between gas stops. And I guarantee you'll get the worst mileage you've every gotten in Baja.
* A working headlight and tail/stop light.
* Hand guards. They need to be the type that mount in two points including the end of the bar. Sure, they're great protection for your hands from errant cacti, but their best feature is that they save your levers from breakage in a fall or tipover.
* New or new-ish chain and sprockets. Why would you risk the ride of a lifetime to a worn out chain or sprocket? You can always set aside and re-install a halfway used set when the new set wears out down the road.
A reasonably quiet exhaust. I know I said reasonably, but you you probably know if others think it's obnoxiously loud or not. If you have an aftermarket pipe, and a "quiet insert" is available-- get one. Notice there's no asterisk here? Don't be the guy that causes me put one here.
Mirrors. We'll do a little time on Mexican asphalt, and you can't be too safe.