The Grain Direction
Do not ever think you can get away with using paper or board with the grain direction not running parallel with the spine. Even paper-backed book cloth has a grain direction you should take into account. (The worst place for a wrong grain direction is in the endpapers of a case bound book. It will be ugly, trust me.) Grain direction is the foundation of bookbinding, and its importance can never be brought up too much.
Least Effort Usually Gives the Best Results
Overworking will result in a book that looks overworked, it's that simple.
A while ago I had a chat with my hairdresser, who is endlessly blown away by crafty people despite being super talented with scissors and hair herself. She complained how all her origami turned out looking terrible regardless of her following each step to a T. I instantly asked whether or not she carefully went over each fold and creased them sharp over and over again. Of course she did! But that's the problem! Sharpening a crease several times can begin to stretch the paper fibres away from one another, and the end result won't look as good as it would if each fold was done well the first time and then left alone. This principle goes for every step of bookbinding, too (and of probably every other craft). Do it well the first time and move on.Paying excessive attention to any area will only result in that area looking overworked once finished. Usually the area you want to keep working on is a problem area, and that's definitely not the area you want to be the focal point of the finished book! So, if it's not outright disastrous, just let it be. And next time, do it better on the first try.
Appropriately Sized Glue Brushes + The Cycle of Glue
If you glue large surfaces using a tiny brush, there's a chance you'll end up with that overworked look. Also, your glue begins to dry where you began before you've finished glueing the rest. Using a brush that's too large on small details will result in too much glue (who would've guessed that?). Too much glue means soaking wet materials and glue in all the wrong places. It's a good idea to keep at least two sizes of glue brushes at hand any given time.
I also have two glue jars - one for sticky glue, one for easy to spread glue. I have big glue pot from which I fill my smaller container, as I don't want to keep that big pot open whilst glueing for long periods of time. It would just dry out for no good reason. If I'm glueing larger surfaces I use slightly thinned down glue, but I like to use that thicker, stickier glue for assembling boxes or sticking down small details without wasting too much time waiting for the glue to dry afterwards. Eventually my sticky glue jar will get too sticky, or just dry out (there's never much in it anyway), but by that time the jar with what's left of the thinner glue will have thickened up enough to become the new sticky jar, and I can replace the old dried out glue with fresh thinned glue. Constant cycle of glue jars. Not sure how professional this approach to glue is, but it works great for me! I use Planatol BB bookbinding glue, but most PVA glues behave the same way.
Folding Sheets for Signatures