How do I choose primers for PCR?

Primers are necessary when running a PCR reaction, which should generally have the following properties:

1. Length of 18-24 bases.
2. 40-60% G/C content.
3. Start and end with 1-2 G/C pairs.
4. Melting temperature (Tm) of 50-60°C.
5. Primer pairs should have similar Tm, within 5°C of each other.
6. Primer pairs should not have complementary regions.