Have a specific plan– something better than "buy some coins and wait til I'm rich."

You should be able to answer these two questions above all else:
1. How much am I comfortable losing?  What's your plan if (actually, when) coins tank? Cut your losses?  Or buy more?
2. What's my exit plan?  Cash out when you've made back your investment and reinvest the rest?  If you never sell, you never make money.  You can hodl BTC forever- or you can cash out some gains at predetermined price levels.  Pick a risk/reward balance you can realistically follow.

Go ahead and write them down.  Put it on a sticky note on your desk.  This way when you wake up to see your holdings are down 30% you don't panic sell.  You don't even think.  You have your plan in front of you.