Well, I was about to go to bed, but I'm laughing so hard now from this thread, that I don't think I can sleep!!!
I'm sorry, but the pic is one worth framing, simply because we've all been there before, and have had to learn thereof...! Dude,,, I'm still laughing at the springs and minivan story too!
Anyway, someone mentioned dropping the rear end down and letting the springs come loose for insertion. I've done that before, and it works well. It's not as much work as it sounds like either. Also, someone mentioned compressing the bag more and putting the cap on it. Well, that works too, with a little effort and maybe a profanity or two...
Either way, get the car up in the air, with the load off the springs, and it will become easier.
Dang!!! I'm still laughing at the other comments and the pic!!!