also, if you're playing with recurring villains, alternate between illusions and conjurations. that way, when they're starting to figure out that you spam illusions a lot, the shit you make start becoming real.
False Theurgy-ing Conjurations to make then look like Illusions is always awesome.
Yeah, I got that skill trick on a cleric/shadowcraft mage of mine, but never really got to use it before the game petered out... I love it, though.
wraiths. they dont make any noise, so you can use the slightly cheaper silent image for simulating them. and you only get a save if you interact with the illusion. Would you run the risk of negative levels just to see if its real? i wouldn't.
I just wanted to reiterate this point that Hijax made. They don't get a save if they don't interact with it, or closely study it. Now what exactly it takes to fulfill that requirement requires a little DM interpretation, but yours seems like he's cool with Illusions and will let you actually use them, rather than be a dick and have the bad guys just ignore them, or always make a save.
It also seems like he's making up a few things as you go, which makes me worry a bit, but so far it sounds like its been in your favor.