Calculated field on a custom form - hints and tricks?

I have recently spent a lot of time trying to create and troubleshoot a calculated field on a custom form - to find out that in some circumstances braces "{ . .. }" are required around some items - although I am not sure how to determine when they are required and when they are not. It is quite off-putting to a relative newby to see an invalid expression message appear as soon as "IF(SUM,,),,)" - both double-clicked from the expression builder - is entered. Does anyone have a collection of hints and/or tricks to use when working with calculated fields? I can't find anything particularly helpful on the HELP site. Thanks in advance Fred