Why is Eric Clapton wrong? He makes it sound like he's telling someone to lay Sally down. ;) If you lay yourself down, the word to use is lie. If you lay SOMETHING down, the word to use is lay.
So, in present tense:
I lie down on bed.
I lay the book on the shelf.
Now, to the tricky part, the past tense (the tense most commonly used in fic):
Severus was too tired to stay up, so he went to the bedroom and lay on the bed.
Hermione laid her book on the shelf and followed him.
Naturally, fiction writing, if done in the past tense, is rarely complete without some past perfect, so:
Severus had lain in bed at the time the authors crashed the door to enter his quarters.
Hermione had just laid the book on the shelf when she heard a noise.
LIE - LAY - LAIN
LAY - LAID - LAID