1. If you ever say the words, "Pure white is just too white." go ahead and slap yourself for me. (Or pinch yourself, because in truth I'm a pincher. Pinch hard.) White is always going to be white. It will always exist, unlike the un-fucking-matchable eggshell ecru that you chose because you're too much of a wimp to go all the way white.
2. Wallpaper. Use sparingly, if at all, and for gods' sake do not wallpaper things like those big baseboards that some older homes are lucky enough to have. I know there's a big flat oh-so-tempting spot between the pretty router-y bits and it seems to be the perfect place to a glue a coordinating stripes & flowers border to go with your flowers & stripes wallpaper, but just don't. Seriously. Go into another room and sit on your hands until the urge passes.
3. Do not add insult to injury. Say, for instance, you have a lovely dining room with a charming little chandelier, lovely hardwood floors, and a bay window overlooking the garden. This perfect room has the singular misfortune of having wallpaper covering the original lath and plaster walls. At the time, it may seem a good idea to cover the wallpaper with something else, like cheap wood-esque paneling. You will go to hell for this, guaranteed.
4. When installing locks, repeat the mantra, "Lefty loosey, righty tighty." Similarly, up is typically the on position for light switches. Sigh.
5. Adding color to your garden can be as easy as sowing a packet of wildflower seeds in a small patch of soil or planting annuals in a nice container. Do not dig up and transplant weeds from the side of the road because you think they are "pretty."
*and there will be future owners (unless you have a gigantic, domicile-sized sati ritual upon your death)