There are quite a few ways of handling this and its best to look at this on a case to case basis. There are some general steps you can follow, though.
If you come across a question with outdated answers then you could do either of the following :
- Write a new answer with updated details
- Put a bounty asking for up to date answers
- Comment under the existing answers regarding the same (both as a means of notifying the OPs as well as putting up a sign post for future readers)
- Suggest an edit to an existing answer covering the updated methodology (not recommended, as such edits have a high probability of getting rejected)
If you get a comment on your own answer, suggesting that its outdated, then you could do either of the following :
- Update the existing answer (if its the accepted answer)
- Write a new answer, and edit the previous one to say that its now outdated.
- Put a bounty asking for up-to-date answers
As every edit brings a post back to the front page, your updated answers will not go unnoticed.
Alternatively, if you want to prevent this issue from happening at all, you can commit to maintaining some of your own answers over time. For best utilisation of your time, just look at the questions you answered which have received the maximum number of views (as these are the ones that most probably turn up in search results), and keep updating them at regular intervals. Even though this may appear to be a thankless job, users who frequent the tag will identify you and appreciate your work (and you will be helping out a whole lot of other people too!)