Hi, Peng-
Well, I don't think there is a way to improve the significance. Basically, you should have one test or comparison in mind, and try it once, and the results you get are what you get. Most people set up their tests to be within a whole brain or gray matter tissue mask, and that's about that.
Something you could check is to make sure that your processing has actually gone well-- was alignment successful for each subject, for example? If you are doing a task-based FMRI study, did you put in the correct stimulus timings? Are you sure you have correctly labelled your subjects and their covariate values? Things like that.
Otherwise, your results are your results. Perhaps gather more subjects for more power?
--pt
