Keep your tire pressure at around 50 psi. Hitting a pot hole or a curb when your tires are not properly inflated can bend your rim, making it wobble. And most of all, having 50 psi will protect you against getting pinch flats. Keeping a spare tube in your saddle bag is a good idea but, again, if your friend gets a flat, and they require a different style tube then that extra tube wont help them. A wise investment.
Nothing works better to prevent flat tires.
We'll deflate your tubes, remove the valve stem, and put 4 ounces of Slime in your front tire and 4 ounces in your rear tire. Then we'll replace your valve stem, and reinflate. You don't even have to stop for a second. They are easy to install and can be transferred from one tire to another. Because most flats occur in the rear tire, install one between the tube and tire of your rear wheel.