This also lowers commissions and fees. Having too many stocks can be worse than too few. For more on how to find such companies, check out the Rule Maker Criteria. Expensive stocks are expensive for a good reason, so don't rule them out simply on that basis. Sell reluctantly and don't panic.
Corrections create opportunities.
You wouldn't want to buy always at the market highs. All stocks eventually have a selloff period, regardless of how great an investment the company may be. Be patient before you add more to your positions. Stocks are the one thing that no one buys when they go on sale. If you believe in the company, buy more when Wall Street has a fire sale or falls out of love with your company.