Don't mine your buildings at all other than the AGT/Obelisk. Drop 4 or 5 in the obelisk/AGT, 4 or 5 outside the back door and then the rest in front of and around the side of the tower. You can almost single handedly defend from FT and APC rushes, leaving more of your team mates to attack. And team mates, if that hotwire buys you 3 mammoth tanks, say thank you.

Also, a tip not just for the techs. When buying a FT (or APC/Buggy/Hummer that you're using for a rush), select your timed c4 before getting in it. If your vehicle gets destroyed, if you're firing at the time you'll probably drop off your C4, then get killed just after. The thing is, most people don't see the C4. I've lost count of how many times I've come running back to see a tank blow up, or hear a boink as I've taken out an opposing engie with it.