careful and repetitive preparation.... good gel coat, several coats of wax each and every time the mould is used, PVA coating and knowing the chemicals/materials you are using in the atmosphere you are in, also pre-heating the moulds is vital.
temperature and humidity even in Canada can play havoc, so in southern parts down there in the USA i know gets mighty warm... usually if the weather is uncomfortable to the average person then its not right for the casting.
adjusting the mix to compensate works, but again familiarity plays a part there.
too hot it boils and can rip the gel coat off the mould, too cold it doesnt expand, over mixed it will not rise... the list goes on... its one of those things that a little bit of knowledge is a dangerous thing, unless you have endless supply of chems and nothing to lose... its a matter of trying it out and learning the products through trial and error.