If I knew the answer to this one I'd grease em on everytime!! However, greasing them on every time probably shouldn't be what you are trying to achieve. Consistantly safe is the way forward. The advice above is sound, most ropey landings don't start over the runway they begin with the approach. Consistancy is the key, once you are into the groove (and the picture starts getting familiar) you are halfway there. Experience is the other bit of sound advice mentioned above, after all it is only by smacking it on a few times that you begin to recognise when things going wrong! Thats what the instructor is for, letting you see when things go wrong without letting them get dangerous!
One tip my instructor gave to me that helped.
Look at the far end of the runway (it provides better height perception for judging the flare).
Don't give up, if the instructor thought you were good enough to take the solo checkride, you can't be too far away!!!!
Best of luck.