How can i beat the knight sacrifice opening and keep the knight?

Is there a way that I can beat this?  I want to keep the knight that is sacrificed, and avoid losing my rook in the process.  The only way I've gotten past this is to not take that tasty knight that is offered up.  Suggestions? (I play as black):

No.  Taking the knight is a forced loss for black.

2...f6 is not very good, it takes away the best square for your knight and exposes your king.  The f pawns are the weakest point in the initial position (only guarded by the king) and also serve as a shield to the king after 0-0 (castling kingside is faster than queenside) so we get the general advice to not move the f pawn in the opening.  When you do, it's often two squares forward to keep the f6 square clear for a knight (or f3 when you're white).

All that said, if you really do want to play 2...f6 here, you can, but you can't take the knight.

A very solid defense is instead 2...d6

shell_knight is absolutely correct. You're playing Damiano's Defense, which even in the best case leaves your position weak, and you definitely can't take the knight without losing quickly...

----------

"Since taking the knight is fatal, after 3.Nxe5 Black should instead play 3...Qe7!^[4] (Other Black third moves, such as 3...d5, lead to 4. Qh5+! g6 5. Nxg6!) After 4.Nf3 (4.Qh5+? g6 5.Nxg6 Qxe4+ 6.Be2 Qxg6 leaves Black ahead a piece for two pawns)^[4] Qxe4+ 5.Be2, Black has regained the pawn but has lost time and weakened his kingside, and will lose more time when White chases the queen with Nc3, or 0-0, Re1, and a move by the bishop on e2."

http://en.wikipedia.org/wiki/Damiano_Defence

----------

It's true the Philidor Defense (2...d6), as shell_knight described, is many times better than the Damino Defense (2...f6), although if you're looking for the *best* 2nd move, 2...Nc6 or 2...Nf6 are your only serious choices, unless maybe you want to take a risk with the Elephant Gambit (2... d5), which some people claim isn't quite sound. Relatively few masters play the Philidor, and virtually no masters play the Damiano unless maybe they're trying to be weird and show off what they can get away with (e.g., http://www.chessgames.com/perl/chessgame?gid=1255180).

It looks pretty similar to the Qe7 line above, but without the kingside weakness brought on by ...f6.  Of course, it's still not good for Black, but if you hate the Ruy Lopez that much, you gotta do what you gotta do!  

Cute variation, The_Coward! Now that's an opening novelty worthy of a name. A similar situation arises in the Petrov Defense:

What the?

2. ... Nc6

It worked for World Champions, it can work for you, too!

Thank you all, and thank you Shell_Knight.  This was really useful!

2..Qe7 is my secret weapon against higher rated players. They'll never expect it.

Too much tactics for me. On 2...f6?! i play 3.Bc4 and on 3...Nc6 4.a3 (to have a safe place on a2). Then i enjoy my permanent slight advantage.

Like someone else said: