huge Mac tip: to choose a startup drive, upon starting up, hold down the option key (alt key on PC keyboards). This will bring up a screen that gives you a choice of startup drives
I've long installed different versions (an old and a new) of the Mac OS on different partitions or different drives in my Mac Pro tower machines. This lets me use newer applications and new features, while I can start up my machine in an older operating system to use older applications that no longer run on newer machines. Right now, for example, I am running OS 10.6.8 Snow Leopard on the same machine that has OS 10.7 Lion running. I can boot into either operating system to use older or newer applications that may not run otherwise.
One huge tip that I came across was the above. I've found that when installing a new operating system, this tip works most frequently, almost without fail, to choose the operating system that you want. Other methods, such as holding down the C key to start up from a DVD, or choosing the startup drive/operating system in System Preferences, only works about half the time. In fact, I remember countless periods of frustration where I was trying to install a new operating system from an Apple DVD, and holding down the C key did not work.
The only other method that works as well as the above is connecting a Mac to another Mac with a Firewire drive, then
Small but incredibly useful applications that I use all the time, which are rarely mentioned in the press:
Carbon Copy Cloner -- I use this utility all the time to back up my drives and to create mirror images of my drives.
SnapZ Pro -- an incredibly useful utility which lets me capture parts of my screen. I can even record video clips up to 2 hours long using this utility; it captures video that is playing on my screen at a choice of screen rates (I use 30 fps) and then converts those thousands of screen grabs into a very watchable and hearable Quicktime video.
Thunderbird, a great email program that lets me organize my emails into different folders. I can archive old folders of emails using the great Import/Export Tools too. Thunderbird is just awesome. Having an email program that grabs emails and then stores them on your laptop, as opposed to using a web-based email program, is great. If I am traveling on a boat, for instance, and I get a wireless signal temporarily, I can download all my emails onto my laptop using Thunderbird, then read and reply to those emails offline. Later, when I am online again, I can send all emails.
I use an iPad to read emails when I am running around town, but I have found that if you only use an iPhone or iPad to view emails, then you will not organize those emails into folders, and you will not remember the history of emails. This might be OK when communicating with friends, but it certainly will not work if you are pounding out a contract with a client, for instance.