With the cactus jump, I would advise people to jump just about the same time as you first hit the cactus.
With the fire jump I usually jump just before going into the flames. It's really hard to see the troll pit once you make it to the fire run. I even knew where it was, but still fell in it. I was lucky & got a fire boost, but most of the time you will die when falling in that hole. If you DO manage to make it through the run, be careful not to fall in the random hole right before the water hole.
Just when you think you're done...there's an invisible wall before the save state. Just double jump from the side to easily avoid it. It's there just as a troll for people who don't know about it.