I bought some ball valves in HD that have a nice feature, and they were the same price as the others. They were Watts valves with union type ends that you can solder on without the ball valve in place, and then insert the valve. The lower mass without the valve body made it easier to get a good flow. Be sure to slip the nuts onto the pipe before soldering the end fitting.
The biggest help to my soldering was when I found Oatey #95 tinning flux. Try some.