Declining the Queen's Gambit by John Cox will have all the answers for you. Excellent book for those looking to play the QGD as Black.
queens gambit declined
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For both given lines the engine says castling and fianccettoing the light squared bishop is good. Also instead of 4. Nf3 Be7 try Bb4 to pin the Knight and possibly double opponents pawns, very common idea. The engine also seems to like transposing into a semi slav setup, so you might want to consider just playing the semi slav or triangle defense as it is also called.
I know I didn't go into a lot of detail, but does that help? If you are looking for more you might want to look at some games masters played with QGD or in general against queens gambit and take what you like from them.
whats the best way to respond (for black) if white plays bf4 or bxf6 in the queens gambit declined?
to be more specific: 1. d4 d5 2. c4 e6 3. nc3 nf6 4. nf3 be7 5. bf4
1. d4 d5 2. c4 e6 3. nc3 nf6 4. nf3 be7 5. bg5 h6 6. bxf6 bxf6