For fleas ~
Lavender oils have been cited as quite effective in flea prevention. Add a few drops of lavender oil to 8 oz water and spray over the Dog. Some people have found it effective.
Eucalyptus Oils used in a similar manner can also be employed as a natural flea prevention. Also try a few drops on bedding etc or even some leaves.
Adding a third apple cider vinegar to two thirds water and poring all over the Dog after bathing has worked well for some. Just dry the Dog lightly and the smell will soon disappear.
A lemon, cut up and left in 8 oz of water for a day has seen good results when then sprayed on your Dog. Preventing fleas in Dogs has a lot of these type of solutions and a lot of the different essential oils mixed with water are said to be very effective.
For ticks ~
Oils such as lavender, lemongrass, eucalyptus,etc can be added to water and applied to the Dog.
There seem to be a lot of natural tick prevention solutions on the market, many simply containing the natural oils stated above. I don’t know if they work well but again you may give them a try. Natural tick prevention really comes down to avoidance. They are much tougher to prevent naturally than fleas.
I really have found little to support these treatments. Frontline medicine, or something like advantix, though chemical based, is definitely the most thorough and effective solution.