There are simple additives that can be used to increase the melt point of your candle and increase the burn time of your candles.
Candle makers have been using Stearic Acid for well over 150 years as a way to increase the melting point of lower melt point waxes. With a melt point of 150 degrees F, it is a fatty acid that is available in two types. Regular Stearic Acid is great for paraffin candles, while its vegetable counterpart Palm Stearic is great for using in Soy Waxes.
Another popular additive is Micro 180, which is a microcrystalline wax. Used anywhere from 2% to 10%, Micro 180 can help eliminate saggy candles in really hot weather.
A word of caution: any additives you introduce to your candles may alter the appearance or burn properties and proper testing must be performed.