The Oxford English Dictionary, regarded as the 'bible' of English form, invariably uses -ize verb-endings, as in publicize, theorize etc. Chambers Dictionary offers both -ize and -ise verb-endings, but with the z version offered first...ie suggesting it is preferred.
All British newspapers, as J says above now use -ise, however, as do most British writers.
As suggested, there are exceptions, which are invariably one or the other and not alternatives...capsize can never be capsise and disguise can never be disguize.