Do stick to the maxim: Buy low and sell high.
But be flexible about it and buy/sell at reasonably low/high prices.
Don't aim to buy in to coins at their lowest lows, or sell at their highest highs. You'll usually end up losing more money this way by chasing prices and making bad decisions. When prices get near their all-time highs or hit very low lows, trading action becomes very erratic and unpredictable. Exchanges get tons of traffic that they often can't handle and you either won't be able to login to your account at all, or won't be able to place orders consistently or in a timely manner.
Avoiding price extremes helps to avoid traffic jams on the exchanges.
Of course, if you happen to get lucky with your timing that's great. Just don't make it your aim.
Do take the emotion out of your trading.
And also get the added benefit of avoiding high-traffic and erratic behavior on the exchanges (see item 1).
Set up limit and stop loss orders well in advance when things aren't crazy and you're not in an emotional mood due to price fluctuations.
Decide ahead of time what prices make sense for your limits and stop losses and place your orders. Most exchanges execute orders as a queue system (first in, first out), so placing your orders well in advance gives you a high likelihood your order will execute near the price your limit is set at.
When setting up your limit prices, don't set a limit at a price like $300. These price points tend to be very crowded and you'll have lots of competition (ie: your order in the queue may be low). Even prices like $299.99 or $300.01 will tend to have higher numbers of orders, so avoid them too.
Set a random price that is close enough to what you want. If you're selling and you want to sell at $300, then sell at a slightly lower price (eg: $299.31) and if you're buying, buy at a slightly higher price (eg: $301.11).
Setting random prices like these highly increase the likelihood of your order executing closer to the price you want in a timely manner.
Do keep some spare cash in your accounts.
Often times I'll see things at very low prices that I'd like to buy, but my money is all tied up in coins that aren't at a good point to sell at. It's time like these I wish I had some freed-up cash in my accounts to pick up on some deals.
Cash is still much less volatile than any cryptocurrency. Keeping your money in a less volatile currency allows you to take advantage of cryptocurrency's volatility when the time is right.
It's also good to take a breather from time to time and not have all your money always locked up. Some times it's quite educational to just watch things for a while and jump in when conditions are good. If you're at all like me, you'll feel great not having to refresh your Blockfolio every 30 minutes all night long. ;-)
Do spread your money across multiple, trusted exchanges.
Exchanges have limits on the amount of the money you can withdraw. You may think the amount of money you have currently invested is far from the exchange's limits, but cryptocurrencies can move fast. And when and if that $7k you invested hits $50k, you may likely want to withdraw it, enjoy your profits and catch your breath for a bit. Spreading your money across exchanges facilitates in not getting stuck by the limits set by a single exchange.
Another reason to spread money across exchanges is to avoid the risk of having money all in one exchange. When prices start jumping around and traffic on the exchanges surge, you may not be able to log in to a given exchange. If one exchange goes down, you'll likely still be able to log in to one or more of the other exchanges and make some of the sales or purchases you were wanting to make. All of your money won't be locked up in one account.
And lastly and perhaps most importantly, there is always the possibility an exchange can be taken offline due to hacking or for legal reasons. Mt. Gox and BTC-e are both good examples of this. Having money in multiple exchanges spreads your risk around and is a much safer bet.
Happy trading!