I have a following problem with walk-behinds. I have a location where first walk-behind is at the bottom of the screen (some rocks etc.) and I set the baseline at the lowest point of the walk-behind area, according to the turorial. But I also have other walk-behinds, hanging from above (which cover only the head of the hero) and those won't work, no matter where I put the baseline the hero is not covered. What am I doing wrong with baselines?
Try to make the "hanging objects" part of the first WBH. Walkbehinds are calculated from the character's FEET, and there is a chance the position of your areas is such that the "overlay" is not detected.
See the image below; with some well-placed walkable areas this works pretty good.
(http://img523.imageshack.us/img523/3032/citytoyshopbackgroundkoqa5.png)
Alternatively, make them an object to cover the character.
If the character is going to be behind the walkbehind at all times, set the baseline to the bottom of the screen. That goes for things on the ground like rocks, or things hanging from the cieling like vines, any walkbehind that the character is always behind, set the baseline to the bottom of the screen.
However, if the player can walk in front of the vines too, then you have to think, like Ghost said, about the characters' feet. If the vines are hanging from the ceiling, set the baseline for their walkbehind at the point on the floor that they would be directly over. Think: If a drop of water fell from the bottom of one of the vines, where would it hit the ground? That's where you need to set the walkbehind. In that case, you'd also want to make a hole in the walkable area at that point so that the player doesn't just pop straight through the vines and instead walks around them.
Of course if the player is always behind the walkbehind, then like I said, just put the baseline as low as it goes. That'll make sure it's always in front.