I know what you mean, it's so frustrating when they wont even land for more than a split second. I spotted my first Marbled White's a couple of weeks ago and spent days trying to find them once they settled.
...One of the tricks is to watch them as it gets to dusk - keeping an eye on one of them (easy to get distracted if there are several) until it roosts. If you're lucky, it will be in a position where you can get to it and with a decent pose, etc. The light will be fading fast (faster than it seems to your eyes because your eye's irises will be dilating to compensate). So you will have to be quick; chances are you'll need a tripod; flash might be OK depending on your equipment and the style of lighting your'e happy with.
You can do the same thing shortly after dawn - when you can get some classic shots of blues or marbled whites resting on grass stems, covered in minute spheres of dew.
Oh - do be careful: it is easy to cause damage or death to roosting insects by trampling at that time of evening.