its always good to CUT OF THE SUPPLIES! specifically start harrassing and burning every single trade caravan going into the marches. You can keep the stuff for your own troops, and since a whole bunch of the FR has that whole interdependant nations thing going, you can severly screw over the people living there. Especially the PCs.
"We need swords!"
"5000 gp please."
"What! We wont pay that, we'd sooner kill you with... uhh... weapons that are no longer available..."
Oh, and another good tactic: send your troops to harry wandering monsters, forcing them in towards the regions towns and cities. The SM's army can hardly deal with an invasion when it's having enough trouble just keeping its people alive. Plus it gives you a good excuse to make the PCs kill stuff.
Orcs have druids right? they are tied to the land, or at least not very civilized?
So have the druids defile all the places they invade. Turn the plants dangerous and alive (assassin vines everywhere). Make all the water just foul enough that it hurts humans, but orcs think it tastes fine.
On that note, have orc scouts bring along druids and clerics, and cast contagion spells on water sources. Its a good way to kill the populace, and really piss the PCs off b/c they cant do anything.
No army is a good one without strong and specialized leaders. Have the majority of the invasion force be regular grunts, but also include a variety of special, really really dangerous leaders. So dangerous in fact that the grunts can't help but follow orders, and will not run. Nothing says freaked out PCs like enemies that will not comprimise.
Use animals to your advantage. They arn't just evil soldiers, they are evil soldiers riding scary dire boar and wyvern mounts.
Above all, if you are going to have a big invasion, keep the invasion force varied. Fighting "orcs" sucks. Fighting mounted enemies with spears and coordinated attack patterns one day, and then getting stalked by a lone but freaky orc the next day, then routing an unorganized orc camp the next day, only to be ambushed by extremly proficicient "orcs with crossbows and magic critters" the next... now that is exiting.