Raise your hand if you haven't performed a weight- and- balance check since your last certificate check ride, or checking out in a new airplane. I have, but I admit that's mostly because older Beech Bonanzas (such as mine) are easily loaded with their center of gravity too far aft. And to boot, because even the main fuel tanks are forward of the center of lift, it's possible to take off within CG limits, but burn your way out of legal aft limits for the airplane.
What I did do when I got the vintage V-tail 10 years ago was to run through a bunch of loading scenarios to see where I would be getting close to the limit. This was not my bright idea, but one I picked up from the American Bonanza Society magazine. I calculated (always with full fuel) how much bulk I could safely load in the baggage compartment if I was alone in the front seat; if I was riding with my wife; or maybe another FAA-standard 180-pounder; and so on. [Alan Klapmeier, co-founder of Cirrus Design, once told me he's "an FAA standard-weight passenger -- with baggage.") From there, I could easily interpolate when I was close enough to the limits to whip out the forms and do the math. You can do this with your airplane, or several airplanes if you rent.
When I looked into becoming an Angel Flight pilot (flying patients and their families for treatment), I learned that every flight had to have a W&B form submitted to headquarters. Curses. But it's sure a good practice.
I also muscled my brain through its limited grasp of Excel spread sheets and devised my own Bonanza weight- and- balance program one day. Now, I just need to plug in the fuel load, the weights at each passenger seat and how much is in the baggage compartment. With this spread sheet, I can determine not only whether I'm within limits for takeoff, but also whether I might have burned enough fuel to compromise controllability on landing at the end of the flight. If you're lucky enough to operate an airplane with a Garmin G1000, Avidyne Entegra or other glass panel, you can work the electronic magic right up front.
The toughest part could be getting your mother-in-law to fess up to where she tips the scales.