Tips 'n' trix
To anti-alias (smooth the edges), resize the image just a few pixels larger or smaller using the b-spline resample filter. The amount needed will be a very small percentage of the original size.
Don't worry about ruining a precious pic. Undo is limited to one, but nothing is permanently changed until you give the order (except for lossless JPG rotation). You can set preferences for warnings on the safe side, and have a dialog pop up for "save" as well as "save as."
Where did I put that??? Watch out when saving, it remembers where you last saved to and that is where the next one will end up unless you browse to a new folder. That is handy, however, if you want to plop a lot of stuff in one place from different sources - like going through a lot of separate photo collections to make a web album.
The best way to fix the "load custom file types" list in the Properties/Extensions dialog is to copy the list from the text field to a text editor (Notepad) and sort it out there, then copy and paste back. Save the revised list as a text file in case you need to restore it after accidentally restoring the defaults!
The amount of some effects is set in the effects browser. When you use the simple effects menu, the effect chosen will be applied at the last browser setting. Therefore, it is best to keep settings at whatever you find most useful. Sharpen can be applied with the keyboard, Shift+S, so it is best kept under ten for most purposes. A low setting can be applied incrementally until the right level is reached.
If you want to save time, learn the keyboard shortcuts for the operations you perform frequently. Copy out the list from Help, edit out what you don't need, print it and stick it on the wall.
When you want to do a drastic readjustment of contrast and/or color, don't try to do it all at once. Crank up values a little bit, exit the dialog, check the image, and then do it again. If it all goes sour, hit Shift+R and start over.