How to reheat without compromising quality

I've never reheated meat or fish to the same flavor and texture that it originally had. I use a small convection oven on a low temperature to bring steaks / ribs / chicken back up to temperature but:

• Even though I manage to keep close to the same doneness of the steaks, they're not as juicy. When I tried reheating them in au jus, I lost the doneness.
• Ribs that fell off the bone the night before come out a tad bit tougher.
• Chicken holds up okay, but it's not as good as when it was first served.
• Delicate fish like dory or sole gets .. well .. not good. Same with shrimp.
• Roasted vegetables come out fine.

In other words, I haven't really gotten better results with more work and care. I don't use my microwave very much after purchasing the small convection oven. I've seen people talking about using a sous vide setup to reheat things; I don't own one and can't speak to how effective it is. There's a convenience factor of leftovers to consider as well (though I'm not saying a sous vide would be any less convenient).

What I try to do is plan the week a bit ahead. If I'm cooking barbeque ribs on Wednesday, then I'll plan on cooking sloppy joe on Thursday with pulled pork using some fresh bell pepper, onion and chili. In other words I try to reuse leftovers rather than just reheat them. So steak, for instance, I'd slice thinly and incorporate it into a pasta. Or I'll dice up some leftover prawns and use them in a salad. I try to always use leftovers in conjunction with something fresh. Why not puree those carrots instead of serving the same thing twice? :)

It's definitely not easy to always plan ahead like that, I'm certainly very guilty of failing to do it and sometimes allowing food to go to waste because I simply don't know what to do with it. It's something you have to think about from the time you put your grocery list together to the time you're putting away the leftovers and doing any prep you'll need for the next day.

I think the most fun is using leftovers in dishes where there aren't that many rules - like tacos, nachos, salads, frittatas, pasta, casseroles and even soups. Some of the stuff I'm most proud of has come out of doing that :)

I'll add a little to Tim's answer, since I have experience with low-temperature water-bath cooking (sous vide).

A water bath is about the gentlest way you can reheat something. You can set your water bath to 125°, 130°F, place bagged (thin) items in the bath—straight from the fridge—and come back half an hour to an hour later to them gently warmed. Except for delicate fish, that will not decrease quality (you may need to sear some things again, as of course bagging it will lose the "crust"). In fact, if you use sous vide the entire way through (put the raw product in a bag, cook it in the bag, chill and store it in the bag, reheat in the bag, and then only open the bag to serve) you keep the quality for a long time—easily a month if done right.

The key thing is that it stays in that perfectly air-tight, air-free bag (and that it's been pasteurized, and kept cold to keep microbes at bay).

Because what no reheating method can do is fix the quality loss during storage, because your protein wasn't wrapped 100% airtight—with all air excluded—as it sat in the fridge. It was drying out via evaporation. Its flavor was changing due to oxidation. That process of actually started (and ran faster!) while it was sitting on the counter during dinner, etc. "Warmed-over flavor" isn't from the rewarming; it's actually due to oxidation, mainly during storage.

You asked for tips: cool leftovers quickly, wrap tightly. And, definitely, take Tim's suggestions for how to use them in alternate dishes.