There's no shortage on articles, books and DVDs on how to get better at
chess.  Often they are written by better players who have established
records as coaches (great players under their tutelage) or stellar results
at world class events.  There may not be as much information written by
players of intermediate strength and how they may have made strides in
playing stength with only a modest amount of time and money devoted to the
game.

I started playing when I was in high school and my start was a rough one. 
I lost nearly all of my USCF provisional rated games and started with a
rating well below 1000 (562 or something).  I worked my rating up to 1300
or so and took about 10 years off before getting back into tournament play
again and my rating has climbed to nearly 1500.  After my 10 year hiatus I
did start to receive some chess lessons via phone and I hope bring up some
of the important themes/ideas that have helped me improve the most.

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For beginners and intermediate players, losses are most often the result of
unforced errors and/or terrible blunders.  Most players in the 1100-1200
category aren't losing tough rook and pawn endgames.  Rather, it seems that
the player who best capitalizes on the opponent's last mistake (usually a
terrible blunder such as a loss of a piece) is the one who wins.  In fact
you can say this is true all levels of chess, but the mistakes in the U2200
section hinge on proper piece placement, the creation of weak squares or
pawns , etc- more subtle features of chess that need much finesse and skill
to score the full point.  Learn to eliminate the huge blunders and you will
find your games lasting much longer.  I believe Vukovich had something he
called a "Compulsion Scale."  Examine ALL checks, threats of mate, threats
to major pieces, threats to minor pieces, and lastly, any moves that could
inflict serious positional harm BEFORE playing your move.

On building an opening repertoire - no area of chess has more books, online
articles and lectures devoted to it than the first 8-15 moves of the game. 
It's also the phase of the game that is blamed the most for one's losses.  I
want to say that Lev Alburt once said that the biggest mistake a tournament
player can make is wandering into a middlegame position that is totally
unfamiliar.  I agree 100%.  So how does one build an opening repertoire and
'toughen' up the the ability play the first 10 moves, or so, correctly?

First off you need some sort of survey that 'grades' what you like and
don't like from the opening.  Someone who has a laid back temperment and
enjoys a long-term struggle, or grind, may favor the Caro Kann as black
instead of an open Sicilian.  Someone who is more risk adverse may enjoy
playing openings that lead to middle game positions that tend to focus on
structural (static) features instead of very dynamic situations where the
play is double edged.

Let me state that the above survey is by no means trivial because you need
to select opening systems (and their variations) that are married to what
you like.  Don't be surprised if it takes several months - one year to
complete this process.  It's a tall order but what you are trying to do is
very thorough and essentially laying out a roadmap that gets you to middle
games that are very comfortable and playable in your eyes.  This is
invaluable to the tournament player because it a) saves time (you can play
the moves quickly since you have some idea where the pieces are typically
placed - more on this later) b) it will calm you down during the game.  You
will feel comfortable and actually look forward to middle game and
ending vs wondering how you will proceed from move 3 or 4.

Let me also state that is FAR more important to understand where the pieces
go and what the normal pawn structures look like vs the memorization of
theory.  Yes, in all opening systems there are some variations that require
very precise play.  In this case memorization may be required just to avoid
losing very early.  However, these are usually isolated cases.  Software
like ChessBase can be a HUGE help in reviewing many games from a given
opening system quickly.  You will quickly see where the best players place
their pieces and why.  It's much better to spend your time reviewing the
openings (and their themes) this way vs some dry opening manual that
doesn't do much more than present tons of variations.

The above idea gels with that Silman (see Reasses Your Chess) tells many developing players.  Find an annotated collection of games from a player you like (Tal, Karpov, etc). Play over the first 10 moves and then cover up the remaining moves and annotations.  Silman advises keep a journal of notes where you trying to guess the moves/plans and compare to the annotations in the book you've
bought.  Silman's idea sort of downplays the opening sequence of moves, but
it's also a good idea to use something like ChessBase to review games
related to your openings - here you are trying to understand why the pieces
are placed where they are and how the piece placement jives with the pawn
structure.  To that end, it's obviously helpful to find books that explain
the positional themes and ideas in WORDS vs 100s of pages of games with
very little explanation.

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Tactics and strategy go hand in hand.  Often the best strategical plans are
defended by solid tactical threats.  Or, solid positional play leads to
situtations where the tactics just take over.  Usually player in very
passive positions don't have much going their way from a tactical standpoint. 

For beginner's and intermediate players I recommend Chernev's Winning Chess
the Basis of Combination in Chess (Du Mont), and the Art of Checkmate
(Renaud).  Understand the basic themes of tactics first - what is a pin, a
double attack, etc.  After this, one can get a hold of a larger puzzle book
- hopefully still grouped by theme and difficulty.  Spending 1-2 hours a
week on tactics is about all you need in my opinion since it's a process
oriented skill.  That is to say, 'just going through the motions' is enough
to keep these skills sharp.

**********

It almost goes without saying that reviewing your games, particularly with
a good teacher or stronger player, will be extremely valuable to one's
improvement.  Nowadays computers are relied on to evaluate a position, but
often all you can see is perhaps where a critical mistake (the main losing
move) was made.  A good teacher will give comments on your opening play and
middle game judgement (and end game play if needed).

With the internet and email, it's easier now more than ever to correspond with a good teacher online.  I suppose it is OK to review a game with some stronger players at your club.  But you want to avoid receiving bad advice from a player who may be just marginally better than you (blind leading the blind so to speak).

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On endgame study.  I must confess that I am not very knowledgeable at the endgame.  Yes, it is important to understand some concepts such as the Opposition and whether or not a king can chase down a pawn from far away, etc it is my opinion that most beginners and intermediate players would be much, much better off firming up the opening repertoire, looking for threats i.e. the Compulsion Scale, and devote 1-2 hours/week towards tactics should come before end game study.  Naturally as one gets better, the player will enter more and more end games and now better end game technique will be just as valuable as anything else.

Wow. That was a lot of typing you did.

Here's a bit of typing from me.

I have thought about all you have said above - many, many times. I'm where you were at some years back, playing Live Chess with other UNDER 1200 players, and that isn't even a USCF strength rating!  Perhaps I would be rated at UNDER 1000 USCF if I played in real tournaments.

Currently, the only parts of chess study that seem to apply to me are: 

1. tactics, 2. checkmates 3. basic endgame techniques and 4. going over the first 5-10 moves to fix bad moves that I make in the opening.

I own many of Silman's books. His endgame book is the only one that has helped so far. I didn't know basic endgame technique, (K + 2R vs. K,  K + R vs. K, K + P vs. K) before reading this book.

However, his books on imbalances and other chess concepts have not made a difference in my rating, yet. At my beginning level, games are won by the most simplists of means - someone blunders and the other player happens to notice. Or, whoever blunders the least, wins. 

My Live Chess rating didn't change until after spending time with Tactics Trainer and the tactics training at Chess Tempo, improving from the 800's to the low 1100's.

My strategy for winning is quite simple: win a minor piece with a tactic, then trade down to a winning endgame.