Don't tell suicidal people not to kill themselves, it's selfish and disrespectful. Offer tangible resources instead. DON'T SAY it gets better -- sometimes it doesn't. You can say you hope it gets better. DON'T SAY it's a permanent solution to a temporary problem -- sometimes it ISN'T, many people deal with depression their whole lives. DON'T act like you know how it feels. Everyone feels things differently. "I care and I am sorry you are suffering" is far better than anything that starts with "You" -- "you'll get better" "you should do _____" "you can't ____."
Show care, show patience, show respect. If you must have an expectation, expect the depression to last forever, and treat it as if it will. The "hurry up and stop being suicidal" attitude is SO FUCKING DEPRESSING.