They sell very cheap, light duffel bags that you can use for this. 

Your pack may be opened for inspection, so you don’t want to seal it up.  The biggest problem with a pack is that all the straps and fasteners can get caught in equipment.  Things (like the straps to cinch up the expansion features) that aren’t really handles get grabbed by baggage handlers and may be damaged or torn off. 

Duct tape over any loose straps, etc.  

Kelty sells this fine backpack duffel which folds up pretty small, but is still very strong and won’t let your pack get messed up:
http://www.shop.com/op/~Kelty_Pack_Duffel_XLarge_Pack_Duffel-prod-33016365-43122557?sourceid=298
 
There are various sizes of it too…….

I never had any problems with my pack.  But before I checked my bag in, I secured all the straps (some of them I tied together), and it worked fine.  British airports are different though.  When I flew out of Edinburgh and London Gatwick, I had to take my bag to a place called “oversize baggage”.  It wasn’t becuase my bag was too large, it was because of all the straps (even though they were all tied and secure).  No other European airport made me do this.

London Heathrow did that too, in 2004. I think the backpack got plastic-wrapped as well. I was worried it might affect whether the backpack actually arrived at the right place on time, but it did with the rest of the luggage.

i found one at mountain equipment co-op for $18. 2 sizes. small and big.

One more note on this…

Be sure you aren’t ordering a RAIN COVER for the pack if you want to check it and not worry about the straps. By definition, these will cover the back part of the pack, but leave the straps totally exposed so you can still wear it. Also, chances are unless it’s a perfect fit, they will fall off in the luggage handling.

You want a BACKPACK DUFFEL style bag, like what Kelty makes. I think Osprey makes one too, but it’s freakin’ enormous.

air canada gave me a big clear plastic bag for my pack – back in 2004

I just got one of those extra large vacuum seal bags big enough to fit a backpack and then roll up in my bag later.  It is clear and should be able to be opened by airport security easily.  It is similar to this with very large dimensions:
http://www.amazon.com/Space-Bag-Jumbo-Size/dp/B00012FF98/ref=pd_bxgy_k_text_b/002-0066218-6982474

Will this work?
 
Dimensions are 48” x 35”
 
For smaller packs the extra large is 39.5” x 26.5”

I don’t think the space sac would be a good idea. I’m not sure if you’ve ever used one of those before, but they are less than durable. I’ve had a hard time keeping them sealed even when trying just to put one in a closet!!

Prob. the best idea is to just get a backpack duffel – but if you want to save some cash – just do the laundry sack idea that someone had.

i dont agree with auher

to save space this is exactly what you use – a RAIN COVER

when on the bag it covers most of the straps. The back strap system you just tie closly together with the same straps and tuck them in, there are only a few straps exposed after the rain cover is on. Lock the hip belt, pull it to the extreme, it all comes very compact and nothgin is sticking out. Secure the rain cover and any straps with duct tape