I always tell people that paying too much for a stock is the cardinal sin.
I don't normally trade stocks. I buy them and hold them 5, 10 or more years. The key to long-term stock ownership is owning QUALITY stocks at low prices. This approach has worked extremely well for me.
While I believe the equity markets are generally overvalued at this time, there are always companies who are undervalued. In essence, just because most companies are overvalued doesn't mean that everything is overvalued.