What is the characterstics of additives that it must posses to make any mixture hypergolic?
The ideal formulation of rocket fuel is a balanced mix of  oxidizer and fuel. A strong oxidizer and a highly flammable compound or strong reducer, which is termed the "fuel", Is the Ideal combination for a fuel that will combust hypergolically when mixed. The compounds in hypergolic fuels are ideally liquids, because fluidity allows for easier mixing. 

As oxidizers, Nitrates work well because the large quantity of oxygen atoms attached to the single nitrogen atom makes for a strong oxdizer. Other compounds, such as Sulfates, Phosphates, Chromates &amp; Dichromates, Chlorates &amp; Perchlorates, Peroxides, and Permanganates also serve as fairly good oxidizers for this reason. However, strong oxidizers do not always require high oxygen content. For example, Chlorine Trifluoride is a very strong oxidizer and is commonly used in many different hypergolic rocket fuel combinations

As far as the rest of the mixture, the fuel, most flammable liquids work well. Aniline, Liquid H2, Hydrazine and it's analogues, Kerosene, Gasoline, and most short-chain alkenes work well with the right oxidizer. An ideal choice when it comes to Hypergolic fuels is fairly small alcohols and ethers, like ethanol, diethyl ether, furfuryl alcohol, and especially Glycerol. It seems that the occasional oxygen atom in the fuel makes it easier and faster to oxidize when the time comes.

In conclusion, Adding a small amount of a stronger oxidizer in solution with your main oxidizer should help spark the reaction if it has trouble getting started. Also, while adding a fuel booster doesn't always work, sometimes adding a little ethanol or glycerol can help get the reaction in motion as well as help it run more smoothly. This is similar to how ethanol is often added to gasoline to increase the efficiency of the burn.

I kind of have an interest in the chemistry of rocket fuels, so if you ever find yourself looking for another answer, feel free to mention me directly.