Solve one problem, quickly
The best lead magnets are small and specific: a checklist for one job, a template someone can fill in today, a calculator that answers one question. A long general guide asks for an evening the visitor does not have. Aim for something a person can use within minutes of getting it.
Our own Knowledge Hub works this way: its free tools, such as the Funnel Gap Finder, show their result first and only then offer to send it by email. Giving the value before asking builds more trust than a locked door.
Make it the first step of what you sell
A lead magnet builds a list of people who want that thing. If it has nothing to do with your product, you get a list of people who will never buy. Pick the first step of the problem your paid product solves, so the people who take the magnet are the people who will need the rest.
Ask honestly
Say exactly what someone will receive, and how often you will email them afterwards. Do not hide marketing emails inside a download, and do not pre-tick consent boxes. The UK Information Commissioner's Office guidance on electronic mail marketing sets out when consent is needed and what it has to look like; it is a clear model even outside the UK.
Deliver at once, and measure what matters
Send the promised thing immediately, in the first email, while the person still remembers asking for it. Then measure two numbers for each magnet: how many visitors sign up, and how many of those later become customers. The second number is the one that tells you whether the magnet works. If sign-ups are fine but nobody buys, the magnet is attracting the wrong people; the note on finding the leak before you buy traffic covers where else to look.