The Tactics Trainer and Daily Puzzle both do a passable job at teaching basic members the principles behind how to punish bad moves made by your opponent.  However, it's the reverse situation I want to cover today.

What principles of play should one follow in order to AVOID ending up on the butt end of a tactical combination in the middlegame?  It seems like I can never stay even, let alone ahead, of my opponents in terms of material, and maybe part of that stems from my utterly beginner knowledge of opening theory (I only know the first three or so moves that give the opening system its name, and have no clue about specific variations).  Even when I'm trying not to be distracted, half the time I miss hanging pieces, pins, forks, skewers and trapped pieces that I would probably be able to see if it was presented in the form of a problem rather than evolving over the course of play.

Doing TT problems trains your pattern recognition. This should work both ways; you'll see the tactical opportunities your opponent gives you and you'll see the tactical opportunities your candidate moves will give your opponent, making you avoid moves which actually do give him tactical shots.

Doing a couple puzzles a day doesn't train it a whole lot though, I'd advise spending the majority of your chess practise time on tactical problems for quite a while.

Playing on longer time controls might also improve your chess (I don't know what time controls you're playing at, but a lot of players play a lot of blitz). Make sure you analyse every move. 

ask yourself the question: is my opponent threatening something next move? Especially with check possibilities and with knights... They are so tricky