How to approach Reti as black?

I don't play against the Reti opening often, but when I do it seems to go poorly. The opening database I looked at doesn't give black the edge in any response to it (except the rare 1. ...h6!?). Making matters worse, it can of course transpose into a billion different things.

So, is there any simple approach to the Reti as black? What do you like to do?

1. Nf3 d5 2. c4 is the start of the Reti. If you do this you are in good shape to transpose to either QGD or Slav with e6 or c6.

Of course, after 1.Nf3 you could play something noncommittal like e6 and see what they are interested in. If they play g3, you can continue with d5 and c5 and Nc6.

QGD or slav.... I will do that next time. Benoni I wish I knew how to play.

The old-school Reti move order 1.Nf3 d5 2.c4 has all but disappeared from GM level chess.  Black does very well with 2...d4.  The Reti structure is usually entered either by 1.Nf3 d5 2.g3 or 1.g3.

The trend against the former these days is to play a system with ...Bg4 whilst delaying ..Nf6.  The latter, obviously, allows Black a lot of options but my preference is to wait with 1.g3 d5 2.Bg2 c6 - with the B committed to g2 it's pretty clear having a pawn on c6 can't be too bad no matter what happens next.

I have had bad experiences with that move order, when the white player was playing Gruenfeld with reversed colors: Nf3 d5 g3 c5 Bg2 Nc6 d4. ..d5 and ..c5 is good but it is better to delay ..Nc6 and first develop kingside.

My preferred setup, because it is ok against all kinds of white setups with g3 is is to play Nf6,(g6,Bg7,0-0) and as soon as white plays g3 i play c6 and d5 (see also GreenCastleBlocks comment), and if possible, e5, another idea, if white plays eg. b3 and Bb2 (double-fianchetto) is b6, Bb7, e6 and c5 etc. If white plays g3 very early, eg. 1.Nf3 Nf6 g3 i sometimes play d5,Bf5,e6, some kind of London setup with Black.

Of course someone has to be ready to play KI, which i am.

It should be noted that 1.Nf3 is a sound opening, do not expect to get advantage with black against it. 

The Reverse Grunfeld is indeed a useful thing to have in one's bag of tricks.  Also certain move orders allow White to transpose to the Tarrasch, e.g. 1.Nf3 d5 2.c4 e6 3.g3 c5 4.Bg2 Nc6 5.cxd5! exd5 6.d4.

So what you're saying is, the Caro-Slav pawn structure works pretty well for black

That's what I tried previously, but clearly I went wrong somewhere. The Caro Slav just works for me! D5/C5 requires a more technique maybe

play the dutch.

After one more move, 5.O-O, Black has to declare his intent.  If Black continues to refuse to take on d4, White will play c4 and get to some kind of Catalan.  For instance, take a look at this variation, which I have studied and happen to like for White:

Yes, 7.dxc5 is the critical line, but i think whites advantage is clear and permanent. I remember analyzing this position for hours trying to equalize, i wasnt successful. Maybe it is defendable in correspondence chess but in a practical (blitz-) game black has to suffer a long time.

This was my mainline, if anyone finds improvements for black, which equalize, please let me know.

Finally i started to search for alternatives to the d5/c5/Nc6 setup.

Thx Pfren, i added your moves to my line. Bb4 looks like a improvement, guess the idea is to take away that annoying Na5, followed by ..f5, when good Nd5 compensates the bishops.