Best solution is the use of a custom format (as recommended by jproj - but see below), or the global solution (as recommended by anka).
However jproj's specific example of a custom format will also display as blank for negative numbers, which might not be what you want. This form of custom format consists of three specifications separated by two semi-colons. The first specification is used for positive numbers, the second for negative numbers, the third for zero numbers. So a more engineeringly-appropriate alternative to jproj's example might be
0.00;-0.00;
Avoid the use of IF() combined with "" as recommended in some of the other posts. I used to use this approach before I discovered the custom format approach, and I know from hard-earned experience that it can lead to subsequent complications when the cell concerned is itself used in later formulae. These problems stem from the fact that the zero contents of the cell can be seen as non-numeric in some contexts.