i have been under the assumption that you have to maintain line of sight on your spells in order to sustain them, but glancing through the sr4, i didn't see anything about that. it only mentions concentrating on it. was it different in previous editions or am i just missing something? or maybe i'm completely wrong; afterall, how would you maintain los on an invis spell other than through astral?

Nope, IIRC once you cast the spell you can maintain it across any distance whatsoever, no LOS required. Other factors may cause you to lose concentration, but LOS isn't one of them.

Also remember that when a sustained spell passes through a ward, it has to make an opposed test of Force x 2 against the Ward's Force x 2.