You could play the Deutch(f5) but I don't know anything about it. If you play d5 and they do anything other than c4 or c3 you could play c5. Or there's king's indian, nimzo indian, gruenfeld etc which all come about from Nf6.
d4 surprise response

Nf6 is what I am playing that doesn't work for me, and that is the most common response.
I like the looks of f5

As a d4+c4 player, I think the responses that would likely throw me off the most are the Dutch (...f5), the Albin Countergambit (...d5+e5), and the Benoni variants (involving an early ...c5). I think most d4+c4 players are pretty prepared to face the myriad of 'normal' black responses: Slav, Semi-Slav, QGD, QGA, and the various Indian defenses.
Of course, Scarblac's response is also very good. Rather than searching for some sideline, why not just stick to your guns (with whatever Indian defense you are playing) and learn how to play it better?

How strong are these opponents? I never lost a tournament game playing the Englund (1. d4 e5) against opponents under 1600 USCF, though it's not strong enough to play now that I'm rated 1700 and playing 1900+ opponents regularly. It gives a fun, open game that they just aren't expecting at lower levels.
Now I play the Classical Dutch, which has the added advantage of working against 1. c4, as well. The basic plan in the Classical line of the Dutch is to play f5, e6, Nf6, Be7, O-O, and then d6 to support a push from e6 to e5. Black often plays things like Qe8 - h5 for a king side attack, Ne4 to prevent white from pushing his e2 pawn to e4, and a5 to prevent white from gaining space on the queen side with b4.
I like to start with e6 before f5 to avoid most of the anti-Dutch lines that some players have prepared, though that risks transposing to the French if white plays 1. d4 e6 2. e4. I like the French, so I don't mind, but it's worth considering if you never play the French.
--Fromper
I recommend the Albin Countergambit. It's not as well-known as the main variations and Black can gain a surprisingly strong attack with the bishops released and a strong pawn wedge.

If you like the french vs. e4 then try the stonewall dutch. No matter what white plays you play 1...e6. If 2.e4 then you're back in a french with 2...d5. If 2.c4 then you've got yourself a stonewall dutch after 2...f5.
http://www.amazon.ca/Win-Stonewall-Dutch-Sverre-Johnsen/dp/1906454078

Budapest defense!!! =) .. If they are not prepared they will be in trouble.. it is 1.d4 Nf6 2.c4 e5?! .. look into it! I have used it otb with great wins!! Or as others have pointed out, the Dutch.. but the Nimzo indian is also great.. Though it takes some preparation..
a surprise weapon will be the budapest defence
though it is a very solid opening
be sure u learn some theory of it
and defeat them good luck
King's Indian; you can't go wrong! The nice part about this opening is that you can generally play the same move order regardless of what white does (unless they're recklessly advancing pawns past the frontier line) and still get a strong position. White will either try to break open your center (unlikely since this leads to an early queen trade), or close the center up with moves like d5. If you've heard of the "pawn pointing" method of attack, you'll see that you have an advantage because your pawns will aim straight to the kingside. Move your queen and rook over there and try to make a break on g2 or h2. White will try to attack on the queenside, so make sure your pawns are adequately defended over there.
why dont you try for Grunfeld defence........????
it is one of the greatest reply comes in the variation in d4 opening ofwhite.
Vishy Anand is the favourite of this defence , so learn and u can have it.

probably BlackKnight_13's head is going around in circles with all the suggestions provided in this thread. I have my own suggestions but there are more than enough ingredients in the pot! However I would like to point out to BlackKNight to stick with whatever he decides to play for some time. Also I would go for a defence which is played by strong players so that it remains a valid defence when you grow stronger.
(my repertoire against 1.d4? It's evolved from Benoni to Nimzo to Benko with a Dutch thrown in against weaker players)
there's a lot of kids in my school's chess club that play d4, and the ones ranked high up always lead me into the same game. I don't like this, does anyone know a good response to throw them off.