Originally posted by: jemcam
Use teflon tape on the threads. Problem solved.
Either that or teflon paste (my personal preference)... But any time you have metal on metal in plumbing, teflon or thread sealant needs to be used.
Also, if you're talking about unions like I'm thinking, you could try putting some plumber's grease on the o-ring so that it makes a better seal. Sometimes the rubber needs a little grease.
If you're talking about compression fittings, you have to tighten pretty damn hard to make a seal. And, when you turn the water back on, if you get a bubble of water, tighten until the bubble stops getting bigger. Compression fittings are tough.. you get one chance to get them right or you gotta buy another.