EDITED: Just typos.
Your "big" [182x218x182] array uses only 57.8 MB of memory. Therefore I would not call it big. And if you install >= 1GB RAM working with such arrays should be done very efficiently in Matlab.
You could use SINGLE or an integer type to save memory, but this does not work, if you need double precision data.
Clearing variables which are not needed anymore is helpful also. But most of all a strict pre-allocation of all arrays is important, because it reduces the fragmentation of the avaliable memory.
But of course installing more RAM and using a 64-Bit system would be the most efficient solution, because this handles the physical source of the memory problem...