One tip I'd like to share maybe not everyone knows:
In between painting sessions I used to intensely (try to) clean the roller. Often that failed and I simply used a new roller because there still was paint left in the roller that had dried out and would screw up your next coat.
However this is still a waste (and if you clean it's a lot of messy work and terpentine to be used) and I heard about this tip somewhere.
First time I tried it I expected nothing from it but.....it works great and have used it ever since:
When you're ready with painting wrap the roller in aluminium foil, do it carefully so it is fully closed.
The next day, simply unpack and roll away again ! The paint left in the roller is still fluent and I have done this cab 6 times with one single roller

I do use fresh aluminium foil each time... else too much risk of holes that will let in air.