try the IDT DNA web site to design your primers

You want to have primer set that:

will have no hearpin structures or the Tm of the structure is bellow your annealing temperature by at list 3-5 degree C;

self-dimmers less than 20%; (same for hetero-dimers)

GC of your primer to be more than 50%

if you design the primer in AT rich area extend the length of the primer to compensate and use "GC clump"

http://www.idtdna.com/pages/scitools

http://www.idtdna.com/analyzer/Applications/OligoAnalyzer/

try the IDT DNA web site to design your primers

You want to have primer set that:

will have no hearpin structures or the Tm of the structure is bellow your annealing temperature by at list 3-5 degree C;

self-dimmers less than 20%; (same for hetero-dimers)

GC of your primer to be more than 50%

if you design the primer in AT rich area extend the length of the primer to compensate and use "GC clump"

http://www.idtdna.com/pages/scitools

http://www.idtdna.com/analyzer/Applications/OligoAnalyzer/

Don't get carried away trying to determine the most accurate Tm; for one thing, the temperature provided by online calculators -whatever the algorithm- does not take into account factors that have a much deeper influence on your optimal annealing temperature, such as the exact Mg++ conc, ATP conc (ATP chelates Mg) and primer conc. of your reaction, and on the other hand, PCR is a very robust technique regarding annealing temperature.

The things Zoia mentions above are far more relevant. The only thing I'd add would be to check for false priming sites.

I am Agree with sir Alenjandro Martin ... That PCR is Robust Technique.

it is Happened to me when i was using the Old Primers it was giving single intense band of 1000 bp product at 55 degree C but when i synthesized new Primers of same sequence they given product of 750 base pairs with Primer Dimer even at Temperature of 60 degree C.