I bought a new rucksack. However, how can I tidy up and hide away the myriad straps so they don't flap about and get caught in things?
The rucksacks that I own, have extra long straps as well. But the manufacturer has provided an elastic band of sorts on the straps to fold them and tuck them within the band (I'll try to post a pic once I get back home). Another option that I have tried is to tuck these longer straps into the side pockets (water bottle holders) and even tie up a lose knot of sorts on themselves to prevent huge swaying straps.
Personally I have never faced any issues as such with longer straps (They don't tend to get stuck up somewhere). It's more of a mental state where I feel the longer straps are kind of a nuisance.
I have used rubber bands, kitchen ties, pipe cleaners, half-broken shoelaces and whatever other pieces of small cord I've been able to place my hands on. All except the rubber bands worked well but none worked for more than a day or two. The rubber bands snapped too easily and I felt a little guilty about leaving little scraps of rubber lying in the wilderness.
I have also gone out with people that dealt with the strap issue by cutting them off! No straps on the outside means nothing to get caught in nasty bush.
Clip together any unclipped compression straps even if you aren't tightening them, just to reduce the length that flaps around. The adjustment straps on the top and bottom of the shoulder straps should generally be kept loose so you can adjust the load throughout a hike depending on terrain. Osprey hip belt straps are often ridiculously long, however, and these are the only ones I ever really try to tuck away into the hip belt so they don't hang down too low.
I've unsuccessfully used elastic bands and duct tape. Elastic bands break or slip off, and duct tape left a sticky residue. I now use Web Dominator, an excellent strap management system using a small plastic clip with shock cord attached. Here's a post describing my experience: http://packinglighttravel.com/travel-tips/health-safety-and-comfort/tame-dangling-straps-web-dominator/
This youtube video offers another option that I have found useful: Managing excess webbing straps on backpacks etc. I purchased what is called in the US "Velcro One Wrap" at a fabric store, around $5 for a 3/4" x 4' roll. Much more than I needed for one backpack, over half is left over.
What I've done before is to place an elastic hair tie on the end of the strap, then just roll the strap up with the hair tie in the center, making a spool around the hairtie. Then you just double up the hairtie repeatedly around the entire spool so that it holds the spool rolled up. The nice thing about this is that it allows you to adjust the length of the loose portion without completely undoing the tie. You can just roll out half a rotation of the spool and then adjust the elastic tie accordingly.