The problems most people have when plastering are:
1) Trying to get it smooth right away - forget it, just get the wall covered as quickly as possible - a 10ft by 8ft wall should take one person no longer than 20 minutes to plaster 1st coat.
3) mixing it too stiff - it needs to be wet
4) taking too long mixing / overmixing / making too stiff then trying to water down with mixer - what many people don't realise is that from the first moment the powder hits the water, they've got about 40 minutes before it's as hard as a dog's head
5) putting little tiny blobs on the wall and trying to spread them out with each motion - it needs long sweeping movements and each trowel full should cover about 4 square feet
so to recap: make sure the wall is ready and you have all tools (clean!) and equipment ready, mix the plaster to a smooth paste slightly thicker than custard, and with no lumps, get the wall covered in long movements, starting at the top and working down to the skirting board, ignoring all lines, marks, lumps, bumps hollows and everything else, then when it's first coated, clean off all trowels etc and wait ten minutes before putting a second thin coat over, this removes 90% of the marks and you shouldn't have broken a sweat.