I've run into several of these, especially the oldest ones with the most use and applied stress. I also have tried gluing many of them back together. Unfortunately, it doesn't hold the best so I usually have to simply resort to buying new pieces. The broken/repaired pieces are useful on occasion, however, as they allow one to do things normally one could not\, and there are other situations on MOC's (and some sets!) where one can bypass the failure. For example, a couple of red "hands" broke on my Torch/Fire Slizer set, so I used them on the hips, where the side sockets are used and not the broken ones. I don't really have any suggestions on how to prevent the breaking of pieces, other than it happens, and usually due to stress. I feel your pain.
My Brickshelf, my Tumblr (it's not as bad as it sounds)
2001: Matoro, Onua, Pohatu, Tahu
2002: Tahnok, Gahlok, Lehvak, Tahnok-Va, Kohrak-Va, Gahlok-Va, Pahrak-Va, Kopaka Nuva, Gali Nuva, Lewa Nuva
2003: Panrahk, Guurahk, Tuurahk, Kurahk
2004: Bordakh (X2), Rorzakh, Vorzahk
2007: Dekar, Defilak, Takadox
2009: Tarduk, Skrall, Tarix, Malum, Mata Nui, Vastus, Stronius, Tuma, Fero and Skirmix
2015: Kopaka, Pohatu, Lewa, Vizuna, Skull Basher
And many random pieces I bought off Bricklink.
1999: Torch/Fire, Scuba/Sub, Ski/Ice, Turbo/City, Amazon/Jungle, Granite/Rock, Electro/Energy, Jet/Judge
2000: Blaster, Millennium/Millennia, Flare, Spark
I also have Jimi Stinger from Breakout.