I think the parts in most kits are designed from first, an ease of manufacturing and 2 an ease of assembly point of view. Weight saving is probably third. Building light means selecting the right wood for the job. You can't use all 4-6 balsa unless you design for it, nor can you use all 12 lb and get a light kit. Each has it's place. Where appropriate, I like to substitute built up for sheets, like the tail and wing tips, etc. Also, if the plane may be heavy in one direction, plan ahead to locate servos and battery to minimize the need for extra weight. If you are fanatic, such things as subbing 1/16 balsa 2-ply crossgrain sheet for 1/8 light ply, or using balsa sheet laminated with a little fiberglass between to save a few grams. I like to use multiple1/16 strips soaked in water and then wrapped around the outline of a wing tip, rudder, or elevator to substitute for some built up or plank designs. It can save weight in a place where it will do much good in the balancing of the model, too. Choice of finishing, and practices in finishing is important, too. When you make a joint, don't fill an existing gap with ca. Make the joint so no gap is present and use the lightest touch of glue needed to bond. Lots of ca or epoxy gaps are heavy.
Be careful what you change. You can make something light enough to fail.
I think the biggest mistake people make in kit mods is when they 'spruce up' some piece. You can make something more rigid so the stress is moved to another part not designed to handle it. Don't expect to make great strides all at once. The very best way to learn about lightening came from talking to the 'old farts' who have been in the hobby for over 25 years. Back when, the power to weight ratios of model engines was much lower than today, so they had to scrimp on weight. You can see that in some old plans from the mags. A plane designed in the 60's for a .60 sized engine may be almost overpowered with a .46 today. And those guys are a vanishing resource. Also, read the mags about electric, indoor, rubber and cmpetition flying. Especially competition. Any competition. Those guys spend time and effort as well as $ to get light and strong, because it gives them the edge. A pilot who owns a world record in pylon is in my club. In a brute power sport, he sweats the weight because at the speeds and g's his models endure every extra ounce becomes almost two pounds of force the airframe must absorb.
It all comes with experience, and attection to detail.
Good luck,
Mike