Encode Gibson homology overlap in only one of the primers (typically in the region that you expect to remain constant in future cloning). This will let you re-use primers between different experiments.
Keep homlogy overlap to between 20-24nt. Adjust length to have at least 52C annealing temperature on SnapGene.
Keep PCR primer binding regions to 17-22nt so that the annealing region has a Tm of >50C (ideally 60C) and a GC content of 35-65%.
Keep all primers to <=60nt since longer primers are both more expensive and have higher error rate.