You can try this:
Disconnect USB.
Turn phone off. Remove battery.
Put battery back on.
Push both volume up and down at the same time. Dont release volumes. Then, connect USB cable (have odin ready)
If that doesn't work, try this:
Disconnect USB.
Turn phone off.
Press and hold volume down and then press power (hold). Hold both until you get into download mode and then connect USB cable.
Important to remember that, with USB cable connected, it won't get into download mode, or at least that's what happened to me but I could fix it following one of the above mentioned methods.
Good luck.
IF you still can't get into DL mode, I suggest you visit
xda-forums.