Soapy water is a surfactant and it is usually used with some neem oil or some other botanical extracts with some silica and that is a good preventative but if a full fledged infestation is taking place something a little more harsh is needed. What insect exactly are you trying to get rid of. this is very important to know to figure out the best approach.
Seems like a weak mixture and a odd thing but it's worked perfectly on my ladies, even a infestation of little ass hole spider mites that neem oil had absolutely no effect on.