For those molded plastic things, get thyself a pair of really good kitchen shears. Then you cut around the packaging, right inside the outside welds. Still get scratched up sometimes, but it’s much much easier than the way I used to do it.
Damn cat food bags. I gave up and use my nice shiny pocketknife on them instead.
Well, I can’t argue with that!
Most of the time I have the same problem with those cardboard pull-strips. Sometimes they work perfectly, but usually not.
Those bags, on the other hand, are pretty easy. Here’s the trick: flip the bag over and open the bottom! You have a narrower crimp and no hole, and you get to lay a little trap for anybody who tries to steal your candy! 
“It’s just so flingin’-flangin’ hard to open!!” 
Ah, modern packaging – the scourge or our times.
Yeah, to stab yourself with out of frustration.
For me it’s DVD wrappers.
Plastic clamshell packaging for action figures and small personal electronics. You can’t open it at all without scissors or a sharp knife, and you have to DESTROY the package to get your item out. So if you have any intention of saving it for any purpose, you’re screwed.
The plastic pouch of brownie mix inside the box. Yes, I make brownies from a box, and yes, I sprayed myself, the counter, and the floor with a fine mist of brownie powder on Sunday night.
(I should’ve just gotten a pair of scissors.)
Mr. Frail swears that M&M bags are deadly, but I think he’s just not using enough nuance in his bag-pulling technique.
I often have difficulty poking the siper straw through the foil target on a juice box, or Mylar[sup]TM[/sup] pouch.
In my favor, I have never hammered it open with my fist and sucked the puddle of juicy goodness directly off the countertop.
<snerk>
The paper bags sewn at the top with the string thingie on the side that says “pull here” and magically it will unsew itself and open nice and neat. Um, not exactly. I’ve had to rip open more bags of birdfeed and potatoes. Hate those things. There’s got to be a trick to it I haven’t mastered.
The plastic encased things also get me. I’ve cut my hand on the sharp edges. I should sue someone over that.
Little plastic pails that are hermetically sealed. I ended up in an emergency room after gashing my hand open trying to cut the notches so I could pry the suck open. Six stitches. Ouch. I bought a pail wrench shortly thereafter.
I believe the heavy gauge plastic containers everybody is bitching about (me too), are called blister paks. They’re the work of Satan i tells ya!
So this company created a nifty tool for opening them. I forget if they were blade based, or heavy shears, or what, but they got good reviews.
This tool? It comes in a blister pak.
Emblazoned on the front, in an ‘as seen on TV’ style logo, it says ‘this is the last thing you’ll have problems getting out of a Blister Pak!’
Those Sick sick bastards.
Yup, really.