Here is my list of the top reasons soldering doesn't work well (looking forward to your opinions on this):
1. Soldering tip has oxidized (turned black) and solder won't stick to it. This happens when the tip is left bare while on--a naked tip will quickly oxidize at hot temperatures. The best advice for preventing this is to glob solder on the tip every time you set the iron down. Weller (high-end iron manufacturer) actually recommends holding solder against a new tip the first time you turn it on so that the solder will melt and cover the tip the very instant the it gets hot enough. One of the best guides out there that emphasizes tip care: Weller's HowTo
2. Bad technique: applying solder to the iron, letting it sit there for a while, and then trying to carry it over to the joint. If you let the solder sit on the iron, the flux quickly boils off (the fumes are from flux, not the lead). And without flux, soldering becomes almost impossible. Flux removes oxidation from metals, and it's crucial because solder won't stick to oxidized metals, and metals oxidize very quickly at soldering temperatures.
3. Not enough heat: A 15 Watt iron is fine for small chips, but any larger connectors or wire bigger than 16 gauge will cause problems. 25W-30W is probably fine for most hobby applications. Is there any risk to getting a 100W iron? Wattage is separate from temperature, right? ... depends on the iron. Watch the video on the next page.
4. Dirty or oxidized parts: Bare copper oxidizes relatively quickly (this is why most components are tin / lead coated), so older parts or bare copper that has been exposed for only a week or two can require a light sanding (pink erasers are great) or stronger flux.
Shameless plug: This instructable is an except from a larger guide with many more pictures found here: www.CuriousInventor.com/HowToSolder. There is also a desoldering guide, a review of the cold heat iron, and advice for choosing solder / flux types. And, of course, a store to buy stuff :)
Thanks for any corrections / feedback.
Step 1: Good Technique
Finally, add solder to the opposite side of the joint. Solder will run towards the heat, so this helps to spread out solder, and also ensures that the components were indeed hot enough for solder to melt and adhere to them.
Step 2: How Much Wattage Do I Really Need?
Answer: Wattage is how much heat can continuously be supplied by the iron. This is, in principle, independent of the tip temperature. On expensive irons, the heat (wattage supplied to the tip) is throttled to maintain a certain tip temperature. The wattage is really a maximum heat output. However, cheaper irons contain a simple heater that is always generating the rated heat, so tip temperature can vary widely. A 15W may only reach 550 degrees F, whereas a 40 Watt can get over 800 degrees.
In general, manufacturers recommend soldering temperatures to be between 650 and 750 degrees F for lead-based solder, and maybe up to 800 degrees for lead-free solder. But why is this if solder melts below 400 degrees? The larger the temperature, the quicker heat will transfer to your parts, so higher temperatures speed up soldering. You could solder at 450 degrees, but this would require some patience. On the other hand, soldering at 1000 degrees would lead to a number of other problems. Parts would be more likely to burn out, the board could be damaged, flux would boil off too fast, soldering tips would burn out quicker, the joints would become more brittle, and the lead would be more likely to atomize and float around in the fumes. All these factors are why many recommend soldering at the lowest temperature possible, but you have to balance the above concerns with the speed that you can solder.