Yo-Yo starting from the pointing upwards position (there is probably a technical term for this). Will try to crack them from the pointing down position next.
Any tips for mastering the flatspin 540 on a DS? Am ending up in a mess with them at them moment, probably due to lack of slack.
Most likely slack, as you pointed out. Timing is important too. Even today I get caught in a tip-wrap if I try to do a super-flat 540.
The DS is supercharged in spin moves and can do 540s at a blistering pace, you may need to practice in really light winds when things are slower and more controlled, go easy on the input as it doesn't take much input to pull off a 540.