I play the QGD as both White and Black. Here's some things to keep in mind when playing the QGD as Black:
1) Move Order - Most of all, don't move that c-pawn too quickly. After 1.d4 d5 2.c4 e6 3.Nc3 Nf6, white has a number of options. If he exchanges with 4.cxd5 exd5, then Black's next move should almost automatically be ...c6. However, after 4.Bg5, best for Black is to play 4...Be7 5.e3 O-O 6.Nf3 and now 6...Nbd7 is the Orthodox variation where the draw ratio is EXTREMELY High. 6...h6 is Black's other option, leading to an "Anti-Tartakower" via 7.Bxf6, or else a Tartakower (7.Bh4 b6) or Lasker (7.Bh4 Ne4). In the Tartakower, the Fianchetto is not automatic. If White fixes a Black pawn on d5 BEFORE Black goes to b7, he will often go to e6 with the Bishop instead, and b6 does still help Black achieve a c5 break. In the Orthodox, there are 2 breaks Black is trying for. He either wants to get in e5 (the preferred one) or c5 to free his position. 6...c6, however, is an error because after 6...Nbd7, what Black does with his c-pawn depends on White's 7th move. After 7.Rc1, Black plays 7...c6 to relieve pressure from the c-file. After 7.Qc2, White is looking at a Kingside attack, and possibly castling Queenside. In this case, Black must be extremely active on the Queenside, and should play 7...c5! in response to that (hence why we don't commit the c-pawn too quickly)
2) In the Exchange Variation (1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.cxd5 exd5 5.Bg5), White has 3 options. Castle Queenside and go for a Kingside pawn storm, Attack the center with Nge2 (instead of Nf3), f3, and e4, or the most popular, go for the Minority Attack. The idea is simple. After cxd5 exd5, only the c-pawn can cover the d5-pawn. Therefore, White needs to play c6, it can't stay on c7. That fact that advancing c5 weakens b7 and d5 makes the pawn such that it NEEDS TO STAY ON c6. This now becomes a target, and White advances the b-pawn. You don't take on b5. When b5 comes, you wait for a trade on c6 (a backwards pawn results) or you check to see if c5 works. If it does, White's b5 move was too hasty. If White goes for this line of attack, you need to know what pieces to try to keep and what to try to trade. Getting the Material (Excluding Pawns) down to QRRN vs QRRN or QRRNN vs QRRNN tends to favor Black. Getting the Material down to QRRN vs QRRB or QRRNN vs QRRBN (In both cases, B represents the LIGHT-SQUARED Bishop) result in a better position for White as Black is not only saddled by the backwards pawn, but also the bad Bishop.
3) Except in the Rare Scenarios where White castles Queenside, Black will usually attack on the Kingside and e-file (especially if White traded c-pawn for e-pawn).
4) In lines where White plays Qc2 instead of Rc1, don't play h6. White doesn't have to move the Bishop if he hasn't castled Kingside. He can play h4!! and if a trade on g5 occurs, the pawn hits the Knigth and the Knight can't move as otherwise, Qh7 is mate!
5) Lastly, Patience Patience Patience! The QGD is not a line where Black blows White out in 20 moves.
I play the Queen's Gambit Declined, mainly because it's simple and I can understand it. But I frequently get into positions where I can't do much with my dark-squared bishop and/or kingside knight because of the pin by white's bishop. I want to know how to alleviate this pressure. I also want to know what are some of the main ideas of the QGD because I just play it because I know the first few moves. I don't know what my goal is when I play it.