Re-use Jars. Jars which used to contain jam, or instant coffee, or pickles can all be re-used in the kitchen. Make sure they are thoroughly wahsed before re-use - you don't want everything tasting of vinegar, do you! Running the jar (and lid!) through the dishwasher a couple of time works well - this also tends to remove the labels, too.
Once a packet or rice - or pasta, or sugar, or nuts, or whatever - has been opened, transfer the remainder to a clean jar. Use a marker pen to label it - so you can tell what's inside easily!
Timing. This is one of the skills of cooking and worth practicing. The objective is to get several things - a main dish, vegetables and a sauce, perhaps - to be all ready at the same time. Of course, you can keep things warm in a low oven, for example, but most food is best when served as soon as it is cooked. Timers which go buzz or feep at the set time help, too.
Warm plates and bowls before serving. Tepid food is generally horrid - it should be either as hot as Hell, or as cold as Charity - a saying I attribute to my Mother. So warming the plates and serving dishes keeps the food hotter for longer. A plate-warmer as part of the oven is great - or you can just position the dishes on top of the cooker, or in the grill area, where the heat from the oven and rings will warm them gently. Don't get them too hot, though, or they might break.