reflexive pronouns with prepositions

Dear Sirs,
from my grammar book I undestood that there should be made a difference when using reflexive pronouns in connection with a preposition. But somehow I don't see the logic in this and where is the difference in these sentences:
a) Look at you!
b) Look at yourself!

a) Would you please shut the door behind you!
b) Would you please shut the door behind yourself!

a) He can't do anything for him.
b) He can't do anything for himself.

a) Please don't tell anybody, keep it to you.
b) Please don't tell anybody, keep it to yourself.

Sometimes it sounds quite ok to have a preposition and a reflexive pronoun. Sometimes it doesn't sound good. Would you please tell me, with which prepositions one can use a reflexive, and when it cannot be use.

Many thanks in advance and regards from Finland,
Kukka-Lotta