OK, so I just started actually enjoying riding on dirt, trails and sometimes no trails, through the boonies. My bike is an old 26 inch wheel, aluminum frame Performance MTB from about 2002, but that's not the issue.
Every time I go out I get a damn, effing flat! So what WTF? What's the best course of action, better tires, thicker tubes, some add-on guard strips inside the tires?
What do you guys do?

First step is to find the cause of the flats. When you install a tube it helps to line up the valve stem with the label on the tire so if you later get a flat you can check the location of the hole in the tube against the tire and know where to look for the cause. If you're getting repeated flats it might be that something is still in the tire or rim and puncturing the replacement tube. If the hole is on the inside surface of the tube check the rim strip to see if one of the spoke holes is partly uncovered or if there's a rough spot on the rim itself. OTOH, if the hole is on the outside check the inside of the tire for something that might be the cause (glass shard, thorn, wire, tire cord). Another possibility is pinch flats caused by hitting potholes or rocks and pinching the tube against the rim - if so then you need more pressure in your tires and/or wider tires.

Check what is causing the flats first, then go from there. Are your tires worn? Go tubeless if you're tired of flats, or there are also Slime-filled tubes, but I have never tried them. What part of San Diego are you in and where do you ride? I'm in SD as well and am always riding Mission Trails or Mt Laguna.

A little slim in the tubes can go a long way against thorns an 40 psi or so helps with pinch flats

Riding off road can present a whole new set of problems not found while riding on the road.
I have a 30yr old Rigid Peugeot MTB and I use WalMart tubes and the old standard 2.1 WTB Velociraptors.
Here is what I do and I never had any problem with flats (but nothing is ever 100% flat proof).

1. Check the inside of your rim for any metal burrs or spokes sticking out.
2 I use strapping tape as rim tape and it is cheap and works great, a couple of wraps.
**Scotch® Reinforced Strapping Tape 8651
3. Check the inside of the tire for any thing that may be puncturing the tube.
4. Use talcum powder on the tube and inside of the tire.
5. Mount the valve stem by the label so you can easily see the air pressure and it is a reference point to find where the flat may have happened.
6. Fill the tube some and place the tube in the tire and mount.
7. I fill up the tire a few psi then peel back the tire bead all the way around on both sides to make sure you are not going to have a pinch flat.
8. I pump up the tire to my preferred psi then check the wheel/tire to make sure it is seated evenly all the way around.
9. I go for a short ride to see how it feels and recheck the tire.
10. I try to do this the day before I go riding (if I can) to let the tire sit overnight and to see if it may go flat overnight.

Hope this helps some because flats su(k...

This is also a good reference:

Park Tool Co. » ParkTool Blog » Tire and Inner Tube Replacement

Some good information. Thanks!

I run Mr Tuffy in all my tires. Slows down thorns and chunks of glass.

in my experience, even with the slime filled tubes (ive tried multiple different brands) it's best to not inflate the tube too close to the highest level psi the tire is rated for, for instance, if the tire says inflate to 40 - 60 psi, then infate it to like 50 and no more.

the reason i make this suggestion is becuase i used to inflate to like 58,59 psi, and when a thorn would make its way thru the tire and puncture the tube, the hole made in the tube would immediately stretch to a size that is too big for the slime stuff to acutally plug.

also i've lately had the case where the inside edge of the tube, that sits over the spoke heads, seperates at the seam, not from a spoke head hitting the tube, but it's just where the seam of the tube is, and it seeems to be rather thin at the seam, and when inflated to the max it seems to help rip this seam apart, and since the slime is being centrifugaly flung to the outside edge of the tube where it touches the inside of the tire, there is no hope for the slime to seal the hole no matter how big on the spoke head side of the tube, i hope this made sense