The old rule - and one still largely followed by The Oxford English Dictionary (the English word-�bible') as well as American usage - was that words derived from classical Greek with the letter zeta in their suffix were rendered �ize' in English...ie retained the �z'.
However, many modern publishers - including �The Times' newspaper, as J says above - prefer the Latinate and more modern �ise' for most such words. Thus, normal British usage has �organise', �apologise' and so on. Certain words - eg �capsize' - are never modified to have an �ise' ending. In the same way, there are others - such as �disguise', �analyse', �circumcise', �televise', �exercise' and �chastise' - which are never given an �ize' ending.
Apart from the exceptions mentioned above, you are basically free to use whichever form - ise/ize - you prefer! Only...be consistent.