Indiana Jones > Antoine de Saint Exupery.
Good point. Plus, I guess, Saint Exupery did drop his plane somewhere over Corsica (and at the end of the war as well, what a bummer!), while Indie is still going strong, if fictional <g>
Seriously though, I guess it has to be a balance of what you use often and what you would have a bit of a hard time finding if you really needed it on the one hand, and mobility on the other. I realized that quite painfully at the age of seventeen, when I'd just started out in more serious photography and was all gung-ho about my equipment. I carried my full kit of the time (heavy east-german SLR plus 3 extra lenses and a tripod, placed in a kit bag totally unsuited for comfortable carrying), up to a medieval castle in wet, muggy summer heat and mid-day sunshine (they don't always go together; but when they do, it's a special kind of hell). By the time I got to the top, I hadn't any energy left to look at the thing, let alone shoot it, and the few pictures I took--half heartedly, and on the way down--were absolutely awful. I remember next to nothing of what I visited that day except for the tops of my boots and the awkward feel of the tripod head in my clammy hand.
After this, it was a 50mm lens on the ole beast--all the way baby. It was three years before I started to use my other lenses again. I took much better pictures and learnt much more when I stopped scrambling around my gear bag for my other lenses all the time, and by the time I fancied using them again, I was much cleverer about knowing which ones to take along for what gig (no sense in taking along the 300mm when I'm shooting my friend's party, lol). not to mention I'd realised those monster lenses were plenty sturdy enough to travel in a rolled up sports sock inside my comfy regular pack
The whole sorry tale taught me that I don't necessarily need more to do the job, and that sometimes the extras are a hindrance rather than a help. Of course, that's not to say you push this in totally the other direction, and go, oh, I don't need any money, I'm just going for a walk and don't intend to buy anything, because what if you want a bottle of water or get bored and decide to take the bus back? But I guess my point is, think about how often you use something; if it's at least once a week, and in a manner you can't predict (ie, not your gym gear that you use three times, but always at specific times, so you can take it along special)--it goes in your bag. Then, think about things you don't use often, but would really be a pain in the neck not to have at some times, for instance because they'd be hard to buy in small quantities or late at night; keep only the smallest of those.
And don't underestimate your own inventiveness; you will
never be able to predict all the situations that might arise, so just take the stuff you use the most, and for the rest, just wing it. It will make you much sharper in the end, and will give you a nice smug little feeling of independance as well, lol.