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Dorfs can dig one level above them as long as the tile below is empty - that is they cannot dig above a table before you move it temporarily.

If you need to dig ceilings taller than 2 floors I can only suggest you make a ladder/staircase to the top of the ceiling then dig down one floor at a time. The last one will be dug partly from above and below - dorfs shouldn't take damage from falling that one floor down.
 
Use the cheapest wood platform/bridges to elevate your position one block up, then just demolishing it when you're finished.
 
The best way is certainly from the top down. Its the fastest and easiest way. Using bridges and ladders can obviously be done - but its a real pain for bigger rooms. I've done it that way with 3 bridges and just leap-frog them down each row. (Because I just didn't want to spend the resource on more bridges). It ended up being a sort of mini game there for a bit trying to keep in sync with my Digger.
:D
 
Start from above and dig down instead of trying to dig up since we usually start somewhere around layer 8 or so.

And the key is to preplan your layout before you start digging as somebody else mentioned.
 
"These backward overdwellers'll never understand proper Dwarven architecture. It ain't nothin' magic to it: We don't dig the ceilings high, we dig the floors low."

-Elbir Burningbeard, First Master Architect, Falsten Stronghold