I usually see suiters make like...velcro loops that go over the arms?
To hold the fur down in place :)

Yeah, I crossposted to the LJ fursuiters community and that was suggested there too. Makes sense, I'll try that...arms are one of the few torso features one can attach stuff to, short of venturing into nipple piercings!

That'd be an easy experiment, good thought! Not everything has to involve hot glue or sewing |)

Yeah, convenient and comfortable clothes are THAT way. :)

The first fursuit head I built had flaps on the front and back that extended about as far as what it sounds like you've done with your head, and I never really had much trouble with them coming untucked (granted, I always wore an Under Armour shirt underneath the partial or bodysuit when wearing it). The problems I ran into with this approach is that it made the suit even more unbearably hot, it made it harder to get the head to air out after use (the fur underneath the chin matted like no other since I had to have the flaps fully spread apart to air out the head interior), and it made putting on the head difficult as you had to tuck the bibs in underneath clothing or the bodysuit. For my second head I've just opted for a shorter neck that rests on top of the suit (though I do need to either extend the neck on the head or actually build a neck on the body suit...or just keep wearing scarves or capes, ha ha ha).

For this case, if tension isn't doing the trick to keep those bibs in place, though, you're right in that you probably need something. The best advice I could give would actually be one of the ideas that you already suggested--velcro on an undershirt. The way I built my current suit's partial sleeves is by putting the soft part of the velcro on the underside of the fur (around shoulder height), then I sewed the hook part of the velcro directly onto the sleeves of a cheap moisture-wicking exercise shirt I picked up at Target. Surprisingly the shirt doesn't really shift at all, despite my crazy arm flailings, and the system works like a charm. Additionally, since the shirt is designed for warm weather exercise it doesn't insulate heat much at all so I barely notice the additional layer of clothing (plus, that helps to keep skin oils and sweat off of the back of your head's fur).

You could easily try this by safety pinning the velcro pieces to the fur and to any 'ol undershirt first to see if you like it and, if it works, commit from there. Just my $0.02! :)

For the REALLY hardcore, Velcro to chest hair. :)

I can see how the partial schema adds problems..you have layers that need to move over each other to simulate actual clothes, but fur is really high friction.

I feel like I've seen some sort of thingie that's an adjustable strap of elastic with light clips at either end...is that the description of a garter belt? It's hard to find things when one doesn't know what they're called!

I'm not sure about the garter belt, but I believe that most partial sleeves do use an elastic rig to hold them up (and I think folks have done something similar with the fursuit head and neck bibs). I tried that on my first suit and hated the way that it worked, hence why I opted for velcro on an undershirt for the second go around. So far it works splendidly! And no need to attach it to chest hair. :P

I'm not sure that the friction is what keeps an overshirt from moving over fur as much as the volume. Fur pile creates a BUNCH of added bulk underneath clothes, especially if you are wearing the same size shirt that you would normally wear, so it ends up filling out the shirt so much that there is little slack left. In theory that alone would be enough to keep partial sleeves in place or, in your case, keep the head bibs from coming untucked. In practice, though, that only worked for me with a very long head bib and long-pile fur, heh. I always had to have some other thing in place to keep the partial sleeves up.

*reads that you would find dressing as Leeloo appealing*
There is not enough bleach for my brain....
Alternatively, that would make for a hilarious art thingy

Appealing, amusing, there's such a fine line. |) Don't worry, there is zero risk of me suprising you like that!

Would magnets or weights work?

I think it would, but I'm hesitant to go that way because I imagine that making it 'hang' funny and behaving strangely when I move around. Maybe I'm wrong though?