Don't throw away manuals and boxes, and keep them in mint condition. Sell games privately instead of trading/selling them to pawn shops and/or used game stores. *Correction* don't sell or trade in games or consoles at all! Don't whip your controllers at the wall at a 100mph when you get pissed off at a game. Don't put your foot through your NES cuz it keeps freezing. Be picky about what games you spend full price on, and have more patience for games to drop in price.
*MOST IMPORTANTLY, DON'T LET YOUR FRIENDS BORROW YOUR GAMES, unless you have deep trust and confidence that they'll take care of your games!*