Lead-free solder melts at a higher temperature than the now-banned lead-based solder. MAPP gas torches burn hotter than propane, making them a better choice for modern solder. Five to 10 seconds of heating with a MAPP gas torch is all that's required before you can feed solder into most 1/2- to 3/4-in. pipes and fittings. Be careful, though. It's easier to overheat a joint with MAPP gas. If the flux turns black and the solder won't flow into the fitting, the joint is overheated.
These small flame-retardant blankets are available at hardware stores and home centers. You hang one behind the joint you're working on to insulate the flammable material and help prevent fires. In a pinch you could use a piece of sheet metal instead. Wetting the area around the soldering job with a spray bottle of water also helps prevent fires. Keep a fire extinguisher handy as a precaution.
Don't feed too much solder into the joint. It's tempting to melt a few inches of solder into a joint as extra insurance against leaking. But excess solder can puddle inside pipes, restricting water flow, and can form small balls that break loose and damage faucet valves. Use about 1/2 in. of solder for 1/2-in. pipe and 3/4 in. for 3/4-in. pipe. Here's a tip. Bend the end of the solder at a right angle, leaving a few inches below the bend. The bend makes it easier to gauge how much solder you've used.
If the threaded fitting is positioned so that solder will run down onto the threads, solder the pipe and fitting at a workbench instead so you can keep the fitting pointed up. If you have to solder a threaded fitting where the solder will flow onto the threads, make sure to wipe excess flux from around the joint after you assemble the pipe and fitting. Extra flux can run down onto the threads, causing the solder to follow it.
Tinning flux works just like standard flux but contains a bit of silver solder powder that melts when heat is applied. The resulting thin layer of solder helps ensure a leakproof joint. Tinning flux is available at most hardware stores and home centers and only costs a little more than standard flux.
Don't try to solder pipes with water in them. When you're repairing or tying in to existing copper pipes, it's common to find a small amount of water in them even after you close the valve and drain the pipes. Soldering a joint in pipes that contain even tiny amounts of water is nearly impossible. Most of the heat from the torch goes into turning the water to steam, so the copper won't get hot enough to melt the solder. Stop the trickle of water with a pipe plug. Push the plug into the pipe with the applicator tube provided. When you're done soldering, dissolve the plug by holding the torch under the spot where the plug is. Plugs for 1/2-in. or 3/4-in. pipe are sold at home centers and hardware stores.
An old trick was to stuff a wad of soft white bread into the pipe to stop the trickle of water temporarily. This works but you run the risk of clogging aerators and valves with the partially dissolved bread.
Cut, flux and assemble a section of pipes and solder them all at once. Soldering one joint at a time is inefficient. Use pipe straps to support the pipes if necessary. Be careful to clean and flux the end of every pipe and the inside of the fittings before assembling them. Then just before you start soldering, press the pipes firmly into the fittings to make sure they're fully seated. Start soldering at one end of the assembly and move methodically from one joint to the next.