Dont scratch over the lyrics of a song.
Less is more - too much scratching detracts from the skill you are demonstrating and can become annoying, as well as making you look like a show-off.
Dont scratch for longer than the song you are playing (unless intentional).
Dont scratch too much with your lower quality vinyl, i.e. thinner records, as they will wear out faster.
Do get professional scratch cartridges and styli, as they are designed with scratching in mind, hence causing less damage to your styli and records.
Do stack multiple records on one deck to allow quicker switches between records.
Do place marks (either with stickers or felt pen) on the sticker in the middle of a record so that you can visually locate the scratch you want lessens the need for headphone use.
Do place your decks in "battle mode" (90 degrees anticlockwise to the norm, i.e. vertically with the tone arm at the top). This avoids contact between you hand and the tone arm and allows the decks to be closer together.
Do use a straight tone arm. Not all turntables allow you to change the tone arm, but some manufacturers do, such as Vestax, Numark and Stanton.
Do ensure your turntables are sufficiently stabilised, i.e. free from shocks, as this can jog the needle.
Do get scratch-intended slipmats.