Leftovers are my best friend!
Everything (and I mean everything!) I make for dinner has to feed us for a least two nights. This way, I only cook every other night at the most. Some of the meals I make feed us for three nights and I have one casserole that will last for four nights. If we get tired of something after the second night, I freeze the remaining portions.
This is a huge time saver not only in terms of cooking, but also planning and shopping (I only have to plan and shop for three meals per week instead of seven). It doesn't take much extra time to double a recipe and it is such a relief knowing I have a meal ready to go on that second night.