When you’re painting a house, it’s said that the preparation is most of the work — and that poor preparation will always result in a poor outcome. I think the same may apply — to some extent — to miniatures. A poor undercoat ruins your figure before you even start. Too much primer, too little, too slick, too grainy. There are a million ways to ruin a figure. Here’s an article on the subject of undercoats.
Matt Fritz has an article on how to keep the paint on plastic miniature wargaming figures.
Here’s an article on priming your figures with artists gesso. I’ve actually been using gesso to prime my figures since the late 1970s. I only use it on 25mm / 28mm figures, though. With my 15s, it is much more efficient to use spray paint.
Junior General has an article on how to solve one of miniature painting’s most intractable problems: getting paint to stick to plastic miniatures.