Make sure to decide which is RS/WS. Sometimes you'll have a preference based on the kind of CO edge you create. You need to be uniform in assembly as you wouldn't want the front(s) of a sweater to have a diff CO edge, on the RS, than the back. Same goes for the sleeves.
I put a little brass safety pin somewhere in the middle of the piece, and on the right side.
I also use little brass safety pins to mark every 10 rows...so I don't have to go back and recount rows a million time. By knowing exactly how many rows I knit to get to (for example) the armhole/armpit shaping...I can knit the other pieces exactly the same number of rows.