I always keep 0.05 bitcoins in my wallet for taking profit from those kind of opportunities. Anyway, sometimes i use those funds for urgently needs, so most times i use that money for myself instead of using it for buying when the price dips. I try to keep always that amount of money in a wallet for doing it.
Last time i bought 0x when the price dropped to $0.17, and i took that opportunity, and i am in profit now.