I got some acidic food in the pot and its starting to look ugly.
Can this pot be saved, or do I need to throw it out?
If it's a small spot you can rub half a lemon on it till it's gone. If the entire inside has darkened you can fill it with water, add several tablespoons of cream of tartar (2 per quart) and boil it for 10 minutes.
You can also buy aluminum cleaner, but the do it yourself is typically cheaper.
In case you're wondering why I'm suggesting an acid for cleaning. It's actually alkali foods that stained your pot, not acidic. e.g. boiling potatoes. Acid actually cleans the spots put there by alkali foods.
The common warning against cooking acidic foods in an aluminum pot is to prevent excessive amounts of aluminum being leeched into your food.
If it's just a matter of darkening and isn't significantly affecting your cooking (i.e. burnt on areas that will cook more quickly) there's no need to worry about it.
I like nice looking pans too but performance is more important than appearance.
As for maintaining a nice clean surface in aluminum and stainless pans, particularly when things have burned or oil and fat have splattered and are creating that "sticky" feel on the edges of cookware, spray it with degreaser or oven spray and let sit for 5-10 minutes before scrubbing with a steel scouring pad. The degreaser works on non-stick pans too but just use a soft nylon pad, not steel for scrubbing it.
When camping with the Boy Scouts our aluminum pots and pans would frequently get burned and stained (cooking on a wood fire, lack of expertise...), but steel wool and detergent would always bring them back to their pristine state. In a pinch, a handful of sand or even plain dirt, as long as it contains enough finely subdivided hard materials, will also work.
When you don't have a delicate/shiny surface to worry about, life is so much simpler.
On cheap steel pots I use a very fine emery cloth that is usable in water. (Sand paper would disintegrate.) I use an old one left on the side for this particular purpose so it is not too rough. You can get very fine grades. My pots are really very clean and I even at times clean the base. Although some pans you shouldn't do this as they have special copper impregnated patterns engraved. Obviously not to be used on non-stick or special surfaces.