Are you annoyed that every time you want to enter an invoice in the Sales Transaction Entry window, you have to manually change the Transaction Type? Here's how to avoid that annoyed . . .
Tools>Setup>User Preferences (GP9 & Previous)
MDGP>User Preferences (GP10)
* click image to enlarge
Set the Default Sales Doc Type to 'Invoice' and there you go. Note that this setting is by user, not global. This allows each user to determine the Sales Doc Type that works best for them.