Use acrylic undercoat and a roller to quickly 'block cover' a dark wall that you're going to paint in a pale colour.
Avoid oil-based paint if poss - takes too long to dry between coats. You can tell which is which by looking at the cleaning instructions - if it says to wash the brushes with water, it's acrylic. With brush cleaner, it's oil-based.
Use good nylon brushes with acrylic - I always bought posh bristle ones thinking posher = better. Not in this case (but usually true!)
Always fill and sand walls using Tetrion - it's so easy to sand and only takes a few hours to dry. REALLY worth doing if you've got pitted and holey walls.
You don't need to sand woodwork very hard - just take the shine off. Again, use acrylic undercoat if you need to change to a lighter colour, it's a much 'denser' white..
You can often get away with painting walls only if you clean the woodwork properly (ie with a scrubber and something that cuts grease)
Definitely use dust sheets to cover the floor - one thing that's hard to fix and looks shoite is paint splashes on flooring.
You can wrap brushes in clingfilm between coats if you do it carefully and tightly. Same with rollers and trays. This means you don't have to clean up all the time which is a massive ball-ache.
Get a paint kettle (little bucket) and pour paint in there from the big pot. Much less likely to spill/splash and can take it up a ladder.
I just bought a proper tall stepladder and it's life changing! No more dicing with death...
Get a pole for the roller - it makes doing the upper wall and ceiling a piece of piss (comparatively).
Using latex gloves makes cleaning up much quicker so more likely to do it in the first place. I hate cleaning up.