I find it helpful to keep a spreadsheet of all the relevant conferences, with columns for the next submission deadline, conference URL, organisation, and organisation URL.
When I find out about a conference too late to submit, I still add it to my spreadsheet. I try to find out if the conference is annual, every two years, or what. I put down a rough guess for the next submission deadline based on the current deadline. That way I'm prepared for the following year.
Also, when I read a paper in my field, I always note where it was published (because the journal or conference might be suitable for my own work). If it's a conference, I add it to my spreadsheet with as much info as I can find.
This is in addition to looking for appropriate mailing lists, as the other answers have covered.