I play the Qd6 line as black, but I'll give my 2p's worth anyway.
I prefer to play Nf6 and a6, but c6 is not uncommon.
A typical start to the game is 1.e4 d5 2.exd5 Qxd5 3.Nc3 Qd6 4. d4 Nf6 5. Nf3 a6.
Now there are many 6th moves. 

A) Bg2-plan
One of the most popular and solid ideas for white is to play g3/Bg2/Bf4/0-0. 
Against this I prefer Bg4, Nc6, 0-0-0. 
After Bf4, Queen goes to d7 or b4 usually, or even d8 if black hasn't castled long. 
Black e5 is an option too, if sufficiently prepared. 
With castling to opposite sides, pawn storms on each side may be on the cards. 

B) Bc4
I often meet 6. Bc4. 
Against this I prefer a6, b5 and Bb7.
In fact, black often wants to play this structure anyway, so he may welcome Bc4 as he gets his plan in with a tempo. 
But with a bishop on the a2-g8 diagonal, it may be a price white is willing to pay. 

C) Bd3/Be2
6. Bd3 is not so good, as Bg4 and pressure against d4 is difficult for white. 
6. Be2 is OK. 

D) What to to with Bc1 and the Queen?
Without g3, white's black-squared bishop usually goes to e3 or g5. 
It may later be played to f4 (to harass black's Queen and increase control of e5) after Qd2. 
With an early Qd2, white may castle long. 

Alternatively, 0-0, Qe2 and Re1 increases pressure on the e-file, and black has to decide what to do with his king and black-squared bishop. 
With control of the e-file, white may be able to play d5 and put pressure on a black pawn on e6 (with Ke8 and/or Be7 seeking cover behind the e6 pawn).  

E) Ne5/Bf4-plan
If 6. Ne5 (with the plan of Bf4 and nasty things happening to Qd6), black has a good move in 6. Nc6.

F) Short cut
A short cut to all CC theory would be to play 3. Nf3 (instead of the standard 3. Nc3), followed by Be2, 0-0 and then c4, d4, Nc3. In addition to cutting down on theory, white gets Nc3 in behind the c-pawn and more central control. Black gets to move too, of course, but I think this is a viable option for white. 

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OK, these were a lot of vague and unstructured ideas, but may give you some food for thought. 
I think g3/Bg2/Bf4 is a good plan, with potential for attack against black's king if Black castles long, and good central control if black castles short. 
So I guess this would be my recommended plan.

But there are several other good 6th moves/plans too (plus the 3. Nf3 short cut). 
In fact, this is a rather flexible opening with no strong theoretical conclusions as far as I know. 

I'd be interested to hear other ideas / preferred structures.