beets - to prevent them from becoming woody/tasteless, keep covering with dirt as they pop out of the ground
carrots - sprinkle row with sand before sowing seeds - the roots grow better and are easier to pull out! OR after placing seeds in rows, carefully pour boiling water over them - the carrots will grow straight
celery - if you want your stalks extra thick, cut an inch off the top of the plant when it is about 4 inches high
cucumbers - pinch off runners for earlier and more abundant produce
peppers - place a layer of tin foil around the base of each plant - the reflected light adds more heat to the plants, keeps the roots cool AND fends off the b.u.g.s.
potatoes - plant deep to avoid hilling (and green potatoes!)
sunflowers - plant a seed at the beginning of every row for an attractive row marker :)
tomatoes - when transplanting, add compost and egg shells to the hole (and around the base once they are planted) - they love the nutrients and the calcium!
I'm already storing up eggshells to use in the garden. Weird, I know :). And, with the assorted tomato cages, eggshells, tin foil, and flowers interspersed, this garden is sure to be a sight :)!
Book: 700 Gardening Tips & Handy Hints, Practical Wisdom from McFayden's Gardening Enthusiasts