Praise be to Allah.
Taking measures and means is not contrary to putting one’s trust in Allah; rather this is something that is enjoined in Islamic teaching, whilst believing that harm and benefit are in the hand of Allah alone, may He be glorified, and that it is He who creates the means and the ends (or cause and effect).
It is essential that there be a real connection between the means and the end (or the cause and the effect), which may be learned from experience, through repetition, from religious texts, and the like.
Moreover, the means or cause must be something that is permissible in Islam, so a praiseworthy and permissible end can only be achieved through means that are permissible and allowed according to Islamic teachings.
Moreover, a person’s approach should be moderate and fall between the two extremes. So he should not neglect appropriate measures altogether, nor should his heart be unduly attached to them. Rather he may take these measures as other people do in their day-to-day lives, then his heart should not be attached to the measures that he has taken; rather his heart should be attached to the Creator, may He be glorified and exalted, the Sovereign and Controller of all things.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Focusing on the means or causes means that the heart relies on that measure and the individual pins his hopes on it and depends on it emotionally, and no created thing deserves that type of attachment or reliance, because it is not independent and does not stand alone; rather there will inevitably be other things that help in that or oppose it. Moreover, if the Creator of measures and causes does not allow them to lead to the (desired) effect, the measures will be to no avail.
End quote from Majmoo‘ al-Fataawa (8/169).
As for the guidelines on taking measures, that varies from one issue to another. Taking measures to deal with mild sickness, for example, is not like taking measures to deal with severe sickness. Taking measures to protect or preserve a valuable and precious item is not like taking measures to protect or preserve an item of little value, and so on.
Taking measures to seek provision is different from taking measures to treat sickness, and taking measures to obtain food and drink is different from taking measures to have a child, bring him up to be righteous and educate him. Thus the measures for anything depend on what it is. As for the guideline on distinguishing between the one who is heedless and negligent and the one who is smart and prudent varies from one issue or situation to another. This is something that people may learn from their own experience and from studying their environment.
And Allah knows best.