If you have Black and White plays 1.e4, the bishop to c4 and Ng5, then you can play the active and surprizing b5 variation.
This line is part of the Two Knights Defence
1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd
Play chess actively now and counter with 5...b5!? instead of 5...Na5 or 5...Nxd5?? (which loses as you know).
1. e4 e5
2. Nf3 Nc6
3. Bc4 Nf6
After 5..b5!? the best move is now the strange looking move 6.Bf1!, but I seriously doubt that White will find this move if he has not studied this b5-Variation.
If White plays another move you have nothing to worry about at all and a good game with lots of tactical chances. If White finds the move 6.Bf1 then he has probably a slight advantage as usual.