I lost a game without making tactical mistakes; how can I avoid this happening in the future? Here's a game of mine from last night.  It was my first tournament game for many years.

(This game is David Kenney vs. Douglas Stones [me] 1-0.  Press F to flip the board.)

[fen ""]
[Event "Bluenose Chess Club Tournament"]
[Site "Halifax"]
[Date "2013"]
[Round "1"]
[White "David Kenney"]
[Black "Douglas Stones"]
[Result "1-0"]

1.d4 Nf6 2.c4 e6 3.Nf3 d5 4.Nc3 c6 5.e3 Bd6 6.Bd3 O-O 7.O-O Nbd7 8.e4 dxe4
9.Nxe4 Nxe4 10.Bxe4 Qc7 11.Re1 Nf6 12.Bc2 Re8 13.Bg5 Nd7 14.Qd3 Nf8 15.
Rad1 b6 16.a3 Bb7 17.b4 Rac8 18.c5 Be7 19.Re4 bxc5 20.dxc5 Rcd8 21.Qe2 
Rxd1+ 22.Qxd1 Rd8 23.Rd4 Rd5 24.Bxe7 Qxe7 25.Be4 Rxd4 26.Qxd4 Nd7 27.Ne5 
Nxe5 28.Qxe5 Qd7 29.Qd6 Qc8 30.b5 g6 31.Bxc6 Kg7 32.Qd7 Qf8 33.Qd4+ e5 34.
Qxe5+ f6 35.Qc7+ Kh6 36.Bxb7 Qe8 37.Qf4+ Kg7 38.Qb4 Qd7 39.Bf3 Qe7 40.g3 1-0


I didn't (seem to) make any tactical blunders, yet seem to have slowly been ground down to a loss via positional play.


  Question: How can I avoid this happening in the future?


It's unclear to me how I could do that; some things spring to mind:


Aim for more tactical openings.
Gain familiarity with the usual endgame pawn structures and piece combinations in the chosen opening.
Study grandmaster games which were decided by positional play.


(Meta-comment:  This question is motivated by the specific game above, but answers should ideally be of general interest, i.e., in the some of the themes will hopefully be useful and interesting category mentioned in this meta thread.)
In this case, the answer is "gain familiarity with the standard strategical themes of the chosen opening".

Black's #1 issue in the Queen's Gambit Declined is what to do with his light-squared bishop, which if ignored can easily get stuck behind the e6 pawn and often a c6 pawn. The two standard ways to free it are


Play ...e5 to allow the bishop to move along the c8-h3 diagonal
Play ...b6 and ...c5 and place the bishop on b7


You kind of got halfway through plan #2, but once White got c5 in, your bishop had no future.

Concentrate on freeing your light-squared bishop the next time you play the Queen's Gambit Declined and you'll be much less likely to get ground down like this. This takes priority over moves like 11...Nf6 (which you just moved back two moves later anyway) and 12...Re8.