1. Subscribe to your favorite calendars
The real power behind iCal is the ability to bring in calendars from others and see the events and activities right along side your own.
2. Publish your calendar
Not only can you subscribe to calendars of others but you can also publish your own iCal so that friends, family and colleagues can know what's on your schedule.
I love this! It does require either access to a WebDAV/CalDAV server such as the one built-in to Mac OS X Server or simply use a MobileMe account. Once you publish it people can either subscribe to it in iCal or Outlook on Windows or just see it on the web like this one.
3. Use Data Detectors to Create Events
If someone sends you an email about an event or appointment there's a good chance that you can create an iCal event directly from that email. Just hover your mouse over the date/time, etc and see if the little arrow appears.
If it does you can have an event created right there on the spot.
4. Use notes in your events
I use the notes field all the time to put in more details about the meeting/event.
For example, who I should ask for once I get there, the best place to park, the address, what they're expecting me to talk about/bring, etc.
5. Set an alarm
You can set one or more alarms for the event and you can set it to alarm as far in advance as you want. The default is 15 minutes.
However, when I put my flight times in I set an alarm for one day/24 hours in advance so that I can check in online. The message pops up on my screen even if iCal isn't running and also pops up on my iPhone so that I don't miss it!
6. Setup Multiple Calendars
I have several iCal calendars that i use on a regular basis. The most common ones are Home and Work.
Since I can turn them on or off as needed this allows me to see just the information I want when I want it. I also setup a different calendar for each of my kid's schools so that I can keep track of things like events and when they have breaks.
7. Turn on the Birthdays calendar
If you have birthdays in your contacts in Address Book, those birthdays will automatically display on your iCal in a special automatically generated Birthday's calendar.
8. Use custom colors
I use colors to determine at a glance which events belong to which calendars.
You don't have to live with the colors assigned by iCal you can customize each calendar to the color of your choice.
9. Invite others via the Address Panel
When I setup meetings/events with friends I can actually have iCal send them an invitation to that meeting in a format that is compatible with most other calendars out there.
For example, when I invited one of my high school alumni buddies to a meeting she told me that it was so neat to have that invite go directly into her Blackberry calendar. Also if they accept you see a little checkmark next to their names!
10. Don't forget about search
Sometimes I forget that the search bar is there and I find myself manually going back month by month looking for an event.
With the search you can just key in what you're looking for and iCal will list all of the matching events.
Bonus Tip: No matter where you are in your calendar you can quickly get back to "Today" by hitting Command-T on your keyboard.
2nd Bonus Tip (unrelated to iCal): You can watch the Apple Keynote Videos on your big screen via Apple TV or take them with you on your iPod or iPhone by simply subscribing to the Apple Keynotes Video Podcast.