Best way to lower a signal voltage
If your connection is one way only and you are fine not insulating your device from your computer then a voltage divider is cheap and perfectly fine. Please note that the divider total resistance may affect speed and power consumption, I'd go for a total resistance not higher than \$10k\Omega\$.

If you need two ways communication you might want to look the mighty web for the MAX232 or its many flavours, this chip is designed precisley to achieve your goal. The MAX will not guarantee any insulation at all but it allows two ways communications.