Sweat a valve
When sweat-soldering a ball valve or gate valve into a copper water line, it is a good idea to solder the valve with the ball or gate in the full closed position.
If the valve is soldered with the gate or ball in the open position and excess solder pools inside the valve, the gate or ball may not fully seat and the valve could "leak through".
This is especially true when the valve is in a vertical pipe run. When soldering a valve in a vertical run, try to pre-fab the valve section before hanging it vertically.
For those vertical runs, an alternate solution is to solder-in threaded adapters and use threaded valves instead of direct-sweat valves.