Painting a portrait is by necessity slow and careful. The tolerances for error are exceedingly small if you are aiming for immediate recognition and I do. That said, simply getting the contours, proportions and skin tones right is not enough in my opinion. Our dear ones have mobile features and a vast array of expressions, some of them highly characteristic.
My modest proposal is rather easier and a heck of a lot more benign that Jonathan Swift's was: USE YOUR CAMERA REGULARLY. Given the high quality of smartphone cameras, virtually everybody has near-constant access to good photography. Use these amazing tools to take lots of close-ups of your beloveds. Group shots are fun but do not take the place of portraiture. It doesn’t matter in the slightest if you plan to have the photo turned into a portrait. What matters is that someone who really matters is documented generously. These crystallized moments will have great meaning in the future. Just remember to regularly download them onto a HD or a DVD which you store somewhere safe off-site.
And what to do if someone you care about seizes up at the sight of a camera? When I wanted to paint a friend whose deer-in-the-headlight expression around cameras bore no relationship to her exuberant personality, I enlisted Jon the Relentless Tease to distract her. Despite herself, she eventually exploded in laughter and SNAP!