PCR primer design guidelines
Optimal primer length: 18-22 bases
Primer annealing temperature (Tm): 55C – 70C
GC content: 40-60%
GC Clamp: The presence of G or C bases within the last five bases from the 3' end of primers (GC clamp) helps promote specific binding at the 3' end due to the stronger bonding of G and C bases. More than 3 G's or C's should be avoided in the last 5 bases at the 3' end of the primer.
Also, avoid primer secondary structures of high melting temperature, self-dimers, heterodimers between primer pairs, repeated sequences and large differences in annealing Tm (>5C) between primer pairs
Primer design guidelines: http://www.premierbiosoft.com/tech_notes/PCR_Primer_Design.html
Tm Calculator for specific DNA polymerase brand products: http://tmcalculator.neb.com/#!/