Which Mountainsmith pack are you using? They make great packs! I have a couple of their Day lumbar packs and several of the smaller Vibe packs. IMO with the bigger ones like the Tour and Day it's a good idea to use the Strappettes. They're small shoulder straps that help support the pack if you're carrying a bit of weight. They work great!
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman