Gambits-always better to decline?

"(the pawn chain is easily undermined),

Exactly:

very fine to    killabeez

In his book "Chess the Easy Way", Reuben Fine said that the best strategy against a gambit is to take the offered material, and then concentrate on rapid development and keeping the King safe, even if that means giving back some or all of the offered material.

Here is a good example to illustrate that idea:

1 e4 e5  2 d4 ed  3 c3 (The Danish Gambit) dc  4 Bc4 cb  5 Bb2 d5!  6 Bd5 Nf6  7 Bf7 Kf7  8 Qd8 Bb4  9 Qd2 Bd2  10 Nd2.  The material is even, and Fine thinks that Black is better here.

c5 is awful. Black can wipe out whites center easily with b6, a5, and e5, while white relieves all pressure on blacks center. White uses a tempo to create a weakness in his own position

I say, IF you know the line of a certain gambit opening well, its weaknesses and how to counter attack it, then go ahead and accept it.

BUT if it's the other way around, it's better NOT to accpet the gambit. You see, gambits are like traps or baits in a battle field. Once you accept it, that would only trigger the attack launch from the "gambiter" using a lot of threat combinations and positional advantage, which would all give it the upperhand.

Gambits are one of the most interesting elements in chess since it appeals to the psychological practicality and emotions of a player. Greed for example. Many players can't help but fall for the "free taste" of one pawn or official held as gambit since it seems as an effortlesly delicious gain and the way it make them believe in the advantage and hold on to that "gain" since it was taken by them without any exchange. But once the gambit is taken, if you don't know how to handle it, it will act as a poison in the gameplan's system. For one or perhaps two pawn/official, the price to pay will be an unstoppable slaughter or a devastating defeat wherein the way to get out (if still possible) is through a very defensive and painful tactical replies.

I'm a gambit fan and patron so I've proven through experience the prowess of those.

After 1.d4 d5 2.c4 c6 3.c5?! b6! 4.cxb6 black has no problems.

I prefer b6 (attacking the pawn chain head-on) because white cannot maintain the chain by normal means. After 3. ... b6! 4.b4 a5! white cannot play a3 to maintain the chain.

However, after 3: ... e5 4.e3 white can maintain the pawn chain in the center, so black has to ask whether the inclusion of e5 and e3 benefits black or white more.

(Maybe if black exchanges and then plays b6 it is best?) The comments about "white cramps black on the queenside" are off-target, since white is practically forced to exchange the "cramping" c5 pawn after black plays b6.

Queens Gambit - Declined and Accepted.

I would sometimes suggest accepting - because players 1600> or maybe even 1800> don't expect their pawn to be taken because they are so used to queen's gambit declined.

Dont stick to lines or standard openings, just try and figure what will happen if you do or do not take the gambit. 
I am currently trying to unlearn openings and actually think about my moves.
I also believe this is very important for a better understanding of the game and picking your own opening repetoire.

i always play the queens gambit when i'm white, and it rarely gets accepted, it pisses me off sometimes

 huh? no offense here but as a gambit player myself I can say that in either ways, accepted or declined,the one that offers the gambit is prepared and knows how to win out that gameplay. That's one of the most basic things one has to acquire anyway if he is to chose a gambit opening. It's a foolishness if you would offer up a gambit and not expect it to be accepted. It doesn't affect much if one seldom find his gambit accepted in previous games, he's still equipped on either ways and can't be taken by suprise by either replies from his opponent.

But of course, this depends still if the one that offers a gambit really knew his line well. If not, well, it could as you've said. But otherwise, accepting gambits generally allows it to gain positional advantage and attack.

 what a coincidence! I do play queen's gambit for white too! yeah, i always wish my opponent would accept it. It does me better :-) probably they've figured that out so they decline more frequently. and that's understandable. anyway no one would give the advantage over to his opponent when he wants the same for his game.

 I believe it was the great Tartakower who quipped, "Book moves are fine, but your own moves are better."  Many class players who spend oodles and scads of time on memorizing opening variations often fall apart when their opponent takes them out of their "book."  Knowing the underlying ideas of the opening is generally more important than knowing specific variations.  (Although as GM Larry Evans has said, knowing the ideas AND the main variations is the ideal scenario.)