I agree with BGBC about buying your own ingredients. I started with Mr. Beer, went on to buying my own ingredients (I'm still extract brewing), and just recently bought my first 5-gallon fermenter. To scale down the standard 5 gallon recipes just multiply everything by 0.4. Rounding off doesn't hurt. This is cooking, not rocket science.
One tip I read here and have used for preserving leftover LME is to pour just a little bit of vodka on top to keep the badness out, put the cap back on, and store it in the refrigerator. It'll last for a fair amount of time, but you should plan on using it in the not-so-distant future. It is a food product containing no preservatives, after all.
Leftover DME is much more stable. Put it in a ziplock bag and store it in the fridge, too. Dry yeast will last for a while, but not all that long. Buy it when you need it and store it in the fridge, then take it out an hour or two before brewing. You can either split the packets in half for your Mr. Beer or just pitch the whole thing. Be sure to reseal the packet tightly if you split it. You can fold the packet over and use Scotch tape. Rehydrating dry yeast first works really well for me. Other people just sprinkle it dry.
Hops should be tightly sealed in plastic bags, double bagged, and stored in the freezer. They have the shortest shelf life of any of the ingredients. Don't buy too much at once.