Slow Down Your Glue
It's hard enough to align and clamp miters without rushing to get it done before the glue begins to set (in five to 10 minutes, and even faster in warm, dry conditions). That's why there are slow-setting wood glues, which give you an extra 10 minutes or so. If you can't find a slow version at your favorite home center, make your own. If you add one part water to 20 parts wood glue, you'll gain about five minutes of working time. The water will also weaken the bond very slightly. So if strength is critical, order slow-setting glue online. Titebond Extend is one common brand.