Great advice on the whole, but I don't agree with the stuff about app widgets.
Widgets don't use a bunch of resources just because they are widgets - I think you could almost say the opposite: the design of widgets allows them to be visible on your desktop without using any resources because the app code that controls the widget only needs to be loaded when the widget changes.
In the end, the resources used by an app widget depend on what it does and how it is designed - same as for any app. If your widget is supposed to go to the network and update some info for you every few minutes then this will drain your battery. However, there are tons of utility widgets that do nothing (and are not in memory) unless they are pressed or one of the phone states they are listening to changes (e.g. a radio is turned on).
Of course, a badly designed app will hurt your battery regardless. Personally I think apps need to get away from creating a custom Application object since these get instantiated whenever the system creates the app's (or app widget's) process, even if it is just to update a widget.