Teflon tape.
From my limited DIY experience with plumbing, when it comes to any screw threads, teflon tape is needed. Even when I changed out shower heads in my bathroom (where the shower head had a washer to seal the connection), I still found it neccesary to use teflon tape to keep the shower head from leaking.
I assume that in your case, we're talking about a pipe that is under constant pressure (main line pipe). IMHO, it is unacceptable for a mainline pipe to have even the smallest of leaks. I also found it quite frustrating to figure out how to get a leakless threaded connection on mainline pipes.
Per instructions you will read from sights like www.irrigationtutorials.com, you should wrap the threads with about 7 turns of teflon tape, hand tighten, and then use tools to give it one more turn.
In practice, I had the most success at creating leakless threaded connections in 1" pipe was to use the 'pink' teflon tape (I believe it's marked as for use with metal pipes), use about 5 turns of the tape (it's much thicker than the white stuff), then "pre-seat" the teflon tape by rubbing it with a cloth (basically I would pretend to be screwing NOTHING on to the tapped pipe). Then hand tighten as much as possble, plus a little more with tools as needed.
But even with this method, It was pretty random on weither it would work or not. But my success when from 50/50 success/failure to 90/10.