1) Always keep a bucket and a spare bin liner in the back of the van, for emergencies when you can't use the customer's khazi. And a couple of bog rolls.
2) Don't make the mistake of relying on a flexi trowel too early in the set. That approach will bite you, sooner or later.
3a) Even if there's good daylight on site, always take powerful LED lights with you, to shine sideways-on for your work,
to check you haven't got ripples, before it's too late and it's set. Could save you some embarrassment. If funds are tight, you can get by with a couple of 100w LED panel lights (about £12 each on eBay). Screwfix also do some OK Erbauer lights on a tripod/monopole, for about £50.
3b) Before you start quoting for paying jobs, shine powerful LED lights on your practice wall in your spare room, to check that you've reached a standard of achieving a wall without significant ripples.
4) Your end result is only as good as your prep - learn how to prepare different surfaces - don't just slap on a coat of PVA and assume it's ready for skimming. Some walls are much more absorbent than others.
If a wall has been stripped of wallpaper, don't be lazy - check the wallpaper paste residue has been dealt with before you start skimming, or you'll regret it.
5) Don't look in the customer's knicker drawer - the odds are you'll get caught!
6, 7 & 8) You probably don't need newfangled tape with holes in it....