Once you master the kitchen basics of cesspool management and not starving, the natural next step is phasing out the type of wasteful habits that would bring tears to the eyes of the wussy heart kid in Captain Planet.
Overdoing it when you're hand-washing dishes
If you have one big sink that doesn't have two sections, it's good to add a container of water in there. So instead of running the water when you're pre-rinsing, you can simply slide your plate in and take it out. You're using the same water to wash the plate. Or you can use your dog for pre-rinsing, which is what I do.
Running an old or half-full dishwasher
If your current dishwasher works great, keep using it, but if you need to replace it, find one that's more efficient. If you have a choice, go with stainless. Plastic interiors don't get as hot, and when plastic heats, it releases toxic gases. That's something you don't want to worry about. And make sure it's Energy Star rated. Also, never run the dishwasher half-empty: always run a full dishwasher.
Running the heater and your oven simultaneously
Once I turn off the oven I leave the door open while it cools down. The fan will stop running and it will warm up the house instead of using other energy. It's not like I turn on the oven to heat my house, but if I am baking, it saves energy.
Shopping without a grocery list
If you go without a list, you're tempted to buy things you didn't need and often people buy doubles, things that they already have. Then in the end they have too much of one thing.
Ignoring the bulk bin
More and more chains are offering bulk bins where you can buy just the amount that you need and use reusable bags to eliminate any packaging waste.
Heating small things in a big oven
Toaster ovens generally use less energy because they're much smaller.
Treating the garbage disposal like a trash bin
Instead of using the disposal, we use a sink strainer then throw it in the compost. I know people tend to be afraid of composting, but it's not as gross as one might think. I was thinking it's too stinky and takes too much time, but there are systems out there that fit everyone's needs. You can even use a worm composter that doesn't smell at all: you just add your scrapes and worms take care of it.
Keeping an empty fridge
An empty refrigerator consumes more than a refrigerator that's full. Once things get cold, if there isn't as much air between each item then it saves energy.
Ignoring your fridge's air filter
Keep your filter clean! We vacuum our refrigerator filter once or twice a year, it makes it much more energy efficient.