Bike bags, carrying weight, related stuff. Short and good.
Any bag makes any bike more useful, and the best bags make a bike look better, too. The fabric should be water-repellent at least. Ours are waterproof.
Where to put the weight and what kind of bag to get.
For the most part, bikes are stable, and bike riders are adaptable, and what this means in the context at hand is that you can put a moderate amount of weight almost anywhere on your bike and in a short time you'll adapt to the load and the bike will feel normal and great.
Once the Physics of Load-Carrying enters the picture, some people fret too much about The Perfect Way to carry weight. It's as though they aren't comfortable unless the the physics tells them they should be; it's not enough to just load up the bike, ride a bit, and get the hang of how your bike handles with a frying pan up front and a sleeping bag in back.
Nevertheless, here are some ways that work for us—
1. For everyday use that's not camping, a basket with a net up front and a biggish saddlebag in back, maybe on top of a rack. The basket is so easy and convenient and fast. You can put a daypack or bag in it, or leave it as a pure basket. It's perfect. The saddlebag in back is for small loose things, or for items you don't want strangers to see, like computers; and it's always good for foul weather.
2. For speedy-sportsy day rides, a small seat bag, that's all. For wallet, phone, keys, repair kit.
3. For touring or camping, just spread the load front and rear, and it doesn't matter whether you keep or lose the basket, use low-rider front panniers or not. Use mid-frame/strap on bags or not. Just try to get a bit more than half the weight in back, and the rest up front. Carrying heavy weights low beats carrying heavy weights high, but if bike riders in China and India can get used to stacking firewood six feet high or toting whole big pigs on top of a rack, you can get used to a highish tent or gallon of water.