I just liberally throw it into sentences where it doesn't belong.
Za dużo / za mało - I say these all the time.
Będę za pięć minut - I'll be five minutes (I actually use this to mean anything between 30 seconds and a couple of hours)
But now I know it can mean behind as well.
Does it go with the instrumental?
I've started to write are you and to, rather than r u and 2. PD really got to me then ;)
Good 4 u.
One criticism of the site is that in the questions about case endings, it asks for the endings by the name of the case, for example:
Instrumental singular masculine animal: śledzi_
which is only good if you know the cases by name rather than by what they do.