The intent here is to list tips and tricks that are not apparent ( maybe just easily or commonly missed ) from the standard documents and tutorials on Eagle. We need contributions, this is just a start.
I like to run auto when it does not finish at 100% I route some stuff on top, or bottom. then run auto again and it will tell how close to 100% you are. You can also unroute a few wires and then re-auto route them.
Hide the top layer
After I manually route on the top layer I sometimes want to rerun the autorouter. By hiding the top layer and then ripping up everything you leave you top layer in place. Show it again and it will remind you where to do the manual routing, and you only have to redo the bottom.
Adding and Removing the Copper Pour
One way to add a copper pour ground plane is to draw a polygon around the whole circuit. Then label it GND. Finally press ratsnest.
A problem I have had is how to remove it if I wish to re-route or make other chages. What I discovered ( eagle 5.3.0 for windows ) was that closing the board and reopening leaves the polygon in place but removes the ground plane. The only other way I have found to get rid of it was to delete the polygon which is a pain if you then have to re-draw it.
Update: You can ripup the pour: click ripup and click on the polygon it removes the pour.
On the bottom I draw some lines around the circuit outside it. When printed either on a carrier sheet or on the toner transfer material this help aligning the board and keeping it centered.
Switch between grids quickly
I've been using Eagle for years but just noticed that you can define grid settings that you use commonly and name them. You do this by right-clicking on the grid icon. Afterward, you can right-click on the grid icon and choose by name.