What kind of guitar is it? Do you have a floating pickup on it, or is it sunken in, and if sunken in, are there one or two pickups?
Floating pickups provides no "grip" on the top plate, therefore it will vibrate like crazy when you hit the right frequency and cause insane feedback. Sunken in sort of "grips" the plate and allows it to vibrate less, and with two of those feedback will hardly be a problem.
There's many solutions for this. My own is just to get to know my amp and my guitar. After a lot of tinkering, I have a setting that produces no feedback whatsoever. I use a fully acoustic archtop with a floater. The key is to know how to level the bass in the amp with the low end sound your guitar produces acoustically. It seems to me that bass frequencies are the bigger problem in feedbacking for archtops.
Other solutions, which I would not spend the money on, is to cover up the holes on the guitar. There are professional foam plugs (I believe Doug's Plugs) that will charge you somewhere around 60 bucks for a pair. You can make your own though with rubber foam. You can also just tape up the holes.
Another more costly option is to get your guitar to a luthier to place something under the wood, between the plates, to keep the plates from vibrating. Different luthiers have their own methods, some place a thick block in there, some place a long, but thin block under the top plate. It's more costy, and it takes away from the acoustic sound of the instrument. I am personally getting a new guitar for more electric stuff, still a carved top archtop, but it's gonna have a huge block in there, plus sunken pickups.
Some people might argue on positioning as well. I don't believe positioning is much of an issue in feedback though. If you're playing you should be able to move freely, and not be limited by feedback issues if you move an inch too much to the left or whatever. That's why I think amp settings go above all, because there will always be a setting where you will sound good, and not feedback.