Try eating a hardy meal 20 to 30 minutes before donating, and eat or drink something relatively high in sugar. That might help.
An important other thing to know is that the minimum time advised between two donations is 3 months. This gap helps blood regain the normal haemoglobin count.so if you are donating frequently without waiting to regain your normal haemoglobin then for sure you will faint and get dizzy.
Another point to check, a blood test must be done to make sure you are not anemic, hematocrit or hemoglobin level is tested to make sure that the loss of blood will not make you anemic, and this check is the most common reason that a donor is inelligible.