Reminding Memory

Guess what comes next in the sequence:

1 2 3 4 5 ? 

The answer is not 6. What is the answer?

When you want to use me, you throw me away. 

When you're done using me, you bring me in. 

What am I?

What goes up but never comes down?

What am I?

What goes up and never comes down?

Which 3-letter word can be inserted into all four lines above to form complete words?

P_ _ _ N T

D E _ _ _ S T

S T _ _ _

C _ _ _ E R