Tile map drawing order techniques?

Im working on a 2D graphics library, and all the blitters are more or less
finished. So now I'm starting to plan the implementation of the tile maps,
and scrolling.

What I am after is an efficient system to sort drawing order in a multi
layer tile map (top down, now isometric). The base layer and transitions can
just be blitted straight, but all the static and dymnamic objects need to be
sorted after Y value, and I want to bve able to have several objects per
tile, with variable y offsets. what is the fastest / most efficient way to
handle this?

I assume a lot of peoiple here have implemented some sort of 2D tile system
at some point, What method did you use to do this? I am not after source
code, just brief descriptions, or hints and advice, or any good web
resources. When I try to google I don't seem to find any info on things like
this.

Thanks for any help.
//Martin