To stay organised calendars are a very important tool. I have tried all sorts of calendars over the years but have always found issues trying to keep different computers in sync with each other let alone other devices like mobile phones.
The only way I have found to get around this is to use Google Calendars. They are available pretty much anywhere, as they are web based as so available from any browser on any computer, but even better they can be made available offline on your computer(s) as well as devices such as the iPhone or iPad (both of which I pretty much rely on for day to day use now).
Another advantage is that you can easily make calendars available to other individuals or even set up a public calendar to share with the world. You can also include calendars other people have shared in with your Google Calendars. A great example of this is the Rugby World Cup 2011 Fixture Calendar.
To get the best use of these calendars though there are a few things that may not be obvious so I will try and go into them in a bit more detail below.