Soldering is a simple, efficient and reliable way to make electrical connections. Those who have never used a good soldering iron think it's normal to have to heat a solder connection for 10-20 seconds to get the components hot enough to melt solder. For most all solder connections, it should take no more than about 2 seconds to heat the component and the solder pad to a temperature that will melt solder.
One reason people are not successful in making good, reliable solder connections (joints) is that they do not properly heat the components which are being soldered. Some people apply the solder only to the tip of the iron. This will most likely result in a 'cold' solder joint. Cold solder joints will almost always fail. The only time that you should apply the solder directly to the iron is when the iron isn't conducting the heat to the parts. When the COMPONENTS being soldered are hot enough to easily melt the solder, you know the components are hot enough to make a good quality solder connection.
Keep the tip of the soldering iron clean. Solder will oxidize and accumulate on the tip of the soldering iron. This oxidized solder will act as an insulator and will prevent good heat conduction to the electronic components. If you use a wet sponge to clean the iron, be very careful. If there is a large quantity of solder on the tip and you press hard on the sponge (compressing it a significant amount), as the iron slips off of the sponge hot solder may be thrown off of the sponge (and it still may be very hot). Using a 100% copper scrubbing pad (like the ones you use for washing dishes) to clean the iron is a better option. I also believe that the wet sponge may significantly reduce the life of the tip.
Selecting a Quality Iron:
If you want a really good, basic soldering iron, I would suggest that you buy a Weller WP35 iron. I have used them for a very long time and they are very reliable. A WP35 (approx $50) is a 35 watt iron which is suitable for almost everything that you will need to do in car audio or electronic repair work. Do NOT buy a $5 iron and expect it to last very long. The tips of cheap irons are usually just bare copper which quickly oxidizes and cannot properly conduct the heat to the components. The tips on the WP35 are steel clad copper which last for months at a time, even when they are used for more than 8 hours a day. The tips are also available in different sizes. The wider thicker tips are more suitable for soldering larger components.
If you have a bit larger budget and can afford to spend about $100 on a soldering iron (recommended if you're going to be doing a lot of soldering), the Weller WES51 is a very good option.
Flux is used to help keep oxygen out of the connection and helps to float contaminants to the surface. It may also help to conduct the heat to the components. Virtually all solder designed for soldering electronics has a core which contains flux. When buying solder for electronics, make sure the flux is not an acid flux. Acid flux is used to solder non electronic components like sheet metal. Flux is also available in a paste form but I rarely use it.
If you have ever tried to remove electronic components from a circuit board (especially multi legged components), you know that it is difficult to remove the old solder without some help. The 2 least expensive, quickest devices to remove the solder are desoldering braid and the desoldering pump. Desoldering braid is simply a flux coated copper braid usually about .1"-.15" wide. To remove the excess solder with braid, you simply apply heat to the braid while the braid is in contact with the solder. The braid will wick the melted solder from the circuit board. The desoldering pump is a device which creates a vacuum to suck up the melted solder. The one I use has a spring loaded plunger which you 'cock' prior to each use. Then when the solder is melted, put the tip of the desoldering pump on the solder joint and release the plunger by pressing (depressing) the release button. For me, it works better if the iron is left on/in the solder while sucking the solder. This will result in a slightly shorter life of the desoldering tip but I have better results. I would recommend buying a 'Soldapult' brand desoldering pump. A large professional quality model is about $30 and will last a very long time. I've used mine professionally for about 5 or 6 years and they are still going strong.
More information and photos for soldering irons and other test equipment can be found on the Basic Amplifier Repair page of the site.