Always - on Print Preview
Some software is, in my experience, not quite WYSIWYG when it comes to the final printed output so I have gotten into the habit of viewing a PDF in preview before I actually send a document to my printer. This has saved many trees.
Apple makes it fairly easy to do this. Just command-P and click on the weird-looking PDF button to get the drop down menu to select "Open PDF In Preview."

I would like to see most every print job automatically open in preview without needing to take my hands off of the keyboard. One can make this happen with a little bit of effort, some help from Folder Actions, and a little piece of free third-party software. Here's how I do it.
First, download and install Cups PDF per the instructions on its home page. This will create a printer and driver that dumps a pdf file into ~/Desktop/cups-pdf/. Set your default printer to CUPS-PDF.
Next, you will setup a folder action on the ~/Desktop/cups-pdf/ to open each file that gets dumped in there in preview. I keep my folder actions scripts in ~/Desktop/Folder\ Actions/, so I put the following applescript (derived from a similar script that ships with Leopard) there.
(* add - new item alertThis Folder Action handler is triggered whenever items are added to the attached folder.
The script will display an alert containing the number of items added and offering the user
the option to reveal the added items in Finder.Copyright © 2002–2007 Apple Inc.
You may incorporate this Apple sample code into your program(s) without
restriction. This Apple sample code has been provided "AS IS" and the
responsibility for its operation is yours. You are not permitted to
redistribute this Apple sample code as "Apple sample code" after having
made changes. If you're going to redistribute the code, we require
that you make it clear that the code was descended from Apple sample
code, but that you've made changes.
*)property dialog_timeout : 5 -- set the amount of time before dialogs auto-answer.
on adding folder items to this_folder after receiving added_items
try
tell application "Finder"
--get the name of the folder
set the folder_name to the name of this_folder
end tell
-- find out how many new items have been placed in the folder
set the item_count to the number of items in the added_items
--create the alert string
set alert_message to ("Folder Actions Alert:" & return & return) as Unicode text
if the item_count is greater than 1 then
set alert_message to alert_message & (the item_count as text) & " new items have "
else
set alert_message to alert_message & "One new item has "
end if
set alert_message to alert_message & "been placed in folder " & «data utxt201C» & the folder_name & «data utxt201D» & "."
set the alert_message to (the alert_message & return & return & "Would you like to view the added items?")
display dialog the alert_message buttons {"Yes", "No"} default button 1 with icon 1 giving up after dialog_timeout
set the user_choice to the button returned of the result
if user_choice is "Yes" then
tell application "Preview"
--fire it up
activate
--open the items
open the added_items
end tell
end if
end try
end adding folder items to
Ctrl (or right) click the cups-pdf folder, Enable Folder Actions, then Configure Folder Actions via the contextual menues seen below.
When you select "Configure Folder Actions" you'll set the script that runs every time a new file is added to that folder to the one above.

Now every time you hit command - P to print in a application, you'll hit enter to print to your default CUPS-PDF printer, which will write a PDF to the cups-pdf folder on your desktop. The folder action will then pop-up this window (for five seconds).

All you have to do is hit enter.
So! Every time I print I hit command - P, enter, wait a sec, enter. Instant preview.
